Exponential perf degradation during substituteGenerics (NTI)

Our moderately sized codebase uses extensive use of the [externs attached](https://github.com/google/closure-compiler/files/1421907/externs.js.txt). Over time, as we've build out these externs, and the corresponding code that uses them, we've seen an exponential degradation in performance, to the point where it's taking literally hours and gigabytes of memory to complete checks-only compilation.

Here's an example stacktrace after pausing in Eclipse once it's taken far too long already:
```
ImmutableMap.copyOf(Map) line: 333
FunctionType.normalized(JSTypes, List, List, JSType, JSType, JSType, JSType, Map, TypeParameters, boolean, boolean) line: 232
FunctionTypeBuilder.buildFunction() line: 191
FunctionType.substituteNominalGenerics(Map) line: 1287
FunctionType.substituteGenerics(Map) line: 1296
ObjectType.substituteGenerics(Map) line: 1587
JSType$ObjsType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 220
ObjectType.substituteGenerics(Map) line: 1584
JSType$NullableObjsType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 218
ObjectType.substituteGenerics(Map) line: 1584
JSType$ObjsType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 220
ObjectType.substituteGenerics(Map) line: 1584
JSType$ObjsType(JSType).substituteGenerics(Map) line: 698
FunctionType.substituteNominalGenerics(Map) line: 1268
FunctionType.substituteGenerics(Map) line: 1296
ObjectType.substituteGenerics(Map) line: 1587
JSType$UnionType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 218
ObjectType.substituteGenerics(Map) line: 1584
JSType$UnionType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 218
ObjectType.substituteGenerics(Map) line: 1584
JSType$UnionType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 218
ObjectType.substituteGenerics(Map) line: 1584
JSType$UnionType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 218
ObjectType.substituteGenerics(Map) line: 1584
JSType$ObjsType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 218
ObjectType.substituteGenerics(Map) line: 1584
JSType$NullableObjsType(JSType).substituteGenerics(Map) line: 698
FunctionType.substituteNominalGenerics(Map) line: 1276
FunctionType.substituteGenerics(Map) line: 1296
ObjectType.substituteGenerics(Map) line: 1587
JSType$ObjsType(JSType).substituteGenerics(Map) line: 698
Property.substituteGenerics(Map) line: 218
NominalType.getProp(String, RawNominalType$PropAccess) line: 414
RawNominalType.getPropFromClass(String, RawNominalType$PropAccess) line: 429
RawNominalType.getProp(String, RawNominalType$PropAccess) line: 458
RawNominalType.getInstancePropDeclaredType(String) line: 478
NominalType.getPropDeclaredType(String) line: 427
GlobalTypeInfoCollector.checkSuperProperty(RawNominalType, NominalType, String, Multimap, Multimap) line: 655
GlobalTypeInfoCollector.checkAndFreezeNominalType(RawNominalType) line: 532
GlobalTypeInfoCollector.process(Node, Node) line: 378
PhaseOptimizer$NamedPass.process(Node, Node) line: 304
PhaseOptimizer.process(Node, Node) line: 230
```

The problem appears to be due to some combination of the usages of:
1. `|undefined` to allow properties to be optional
2. `Array<...>` generic types
3. The depth of the tree of types being defined in the externs

Performance is improved by breaking up the structure of these externs type trees through changing various arrays to `Array<*>` and putting in casts at point of use.

I've created a very simple [example in the debugger](https://closure-compiler-debugger.appspot.com/#input0%3D%250A%252F**%2520%2540abstract%2520*%252F%250Aclass%2520X%2520%257B%250A%250A%2520%2520%252F**%250A%2520%2520%2520*%2520%2540return%2520%257B%253FHighcharts.Options%257D%250A%2520%2520%2520*%2520%2540abstract%250A%2520%2520%2520*%252F%250A%2520%2520getOptions()%2520%257B%257D%250A%250A%257D%250A%250Aclass%2520Y%2520extends%2520X%2520%257B%250A%2520%2520%252F**%2520%2540override%2520*%252F%250A%2520%2520getOptions()%2520%257B%250A%2520%2520%2520%2520return%2520%257B%250A%2520%2520%2520%2520%2520%2520chart%253A%2520%257B%2520animation%253A%2520true%2520%257D%252C%250A%2520%2520%2520%2520%2520%2520title%253A%2520null%252C%250A%2520%2520%2520%2520%2520%2520subtitle%253A%2520null%250A%2520%2520%2520%2520%257D%253B%250A%2520%2520%257D%250A%257D%250A%26input1%26conformanceConfig%26externs%3D%252F**%2520%2540constructor%2520*%252F%250Avar%2520Element%2520%253D%2520function()%2520%257B%257D%253B%2520%252F%252F%2520not%2520sure%2520why%2520debugger%2520doesn't%2520know%2520about%2520this%250A%250A%250Avar%2520Highcharts%2520%253D%2520%257B%257D%253B%250A%250A%252F**%250A%2520*%2520%2540param%2520%257BHighcharts.HighchartsOptions%257D%2520globalOptions%250A%2520*%252F%250AHighcharts.setOptions%2520%253D%2520function(globalOptions)%2520%257B%257D%253B%250A%250A%252F**%250A%2520*%2520%2540typedef%2520%257B%257B%250A%2520*%2520%2520global%253A%2520(Highcharts.GlobalOptions%257Cundefined)%250A%2520*%2520%257D%257D%250A%2520*%252F%250AHighcharts.HighchartsOptions%253B%250A%250A%252F**%250A%2520*%2520%2540typedef%2520%257B%257B%250A%2520*%2520%2520useUTC%253A%2520(boolean%257Cundefined)%250A%2520*%2520%257D%257D%250A%2520*%252F%250AHighcharts.GlobalOptions%253B%250A%250A%252F**%250A%2520*%2520%2540constructor%250A%2520*%252F%250AHighcharts.SVGRenderer%2520%253D%2520function()%2520%257B%257D%253B%250A%250A%250A%252F**%250A%2520*%2520%2540type%2520%257BObject%253Cstring%252C%2520function(number%252C%2520number%252C%2520number%252C%2520number)%253AArray%253C(string%257Cnumber)%253E%253E%257D%250A%2520*%252F%250AHighcharts.SVGRenderer.prototype.symbols%253B%250A%250AHighcharts.seriesTypes%2520%253D%2520%257B%250A%2520%2520%2520%2520wordcloud%253A%2520Highcharts.WordcloudType%250A%257D%253B%250A%250A%252F**%250A%2520*%2520%2540type%2520%257B%257B%250A%2520*%2520%2520placementStrategy%253A%2520*%252C%250A%2520*%2520%2520deriveFontSize%253A%2520function()%250A%2520*%2520%257D%257D%250A%2520*%252F%250AHighcharts.WordcloudType%253B%250A%250A%252F**%250A%2520*%2520%2540param%2520%257BElement%257D%2520element%250A%2520*%2520%2540param%2520%257BHighcharts.Options%257D%2520options%250A%2520*%2520%2540return%2520%257BHighcharts.Chart%257D%250A%2520*%252F%250AHighcharts.chart%2520%253D%2520function(element%252C%2520options)%2520%257B%257D%253B%250A%250A%252F**%250A%2520*%2520%2540typedef%2520%257B%257B%250A%2520*%2520%2520%2520%2520%2520%2520chart%253A%2520(Highcharts.ChartOptions%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520credits%253A%2520(Highcharts.CreditsOptions%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520legend%253A%2520(Highcharts.LegendOptions%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520pane%253A%2520(Highcharts.PaneOptions%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520plotOptions%253A%2520(Highcharts.PlotOptions%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520series%253A%2520(Highcharts.SeriesOptions%257CArray%253CHighcharts.SeriesOptions%253E%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520subtitle%253A%2520(%253Fstring%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520title%253A%2520(%253Fstring%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520tooltip%253A%2520(Highcharts.TooltipOptions%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520xAxis%253A%2520(Highcharts.AxisOptions%257CArray%253CHighcharts.AxisOptions%253E%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520yAxis%253A%2520(Highcharts.AxisOptions%257CArray%253CHighcharts.AxisOptions%253E%257Cundefined)%252C%250A%2520*%2520%2520%2520%2520%2520%2520zAxis%253A%2520(Highcharts.AxisOptions%257CArray%253CHighcharts.AxisOptions%253E%257Cundefined)%252C%250A%2520*%2520%257D%257D%250A%2520*%252F%250AHighcharts.Options%253B%250A%250A%252F**%250A%2520*%2520%2540typedef%2520%257B%257B%250A%2520*%2520%2520animation%253A%2520(boolean%257Cundefined)%252C%250A%2520*%2520%2520type%253A%2520(string%257Cundefined)%252C%250A%2520*%2520%2520events%253A%2520(Highcharts.EventsOptions%257Cundefined)%252C%250A%2520*%2520%2520alignTicks%253A%2520(boolean%257Cundefined)%252C%250A%2520*%2520%2520zoomType%253A%2520(string%257Cundefined)%252C%250A%2520*%2520%2520inverted%253A%2520(boolean%257Cundefined)%252C%250A%2520*%2520%2520polar%253A%2520(boolean%257Cundefined)%252C%250A%2520*%2520%2520options3d%253A%2520(Highcharts.Options3D%257Cundefined)%252C%250A%2520*%2520%2520margin%253A%2520(number%257Cundefined)%252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but it doesn't show any obvious performance problems. Probably it needs a larger real-world codebase, which I can't submit.

Externs in full, attached: [externs.js.txt](https://github.com/google/closure-compiler/files/1421907/externs.js.txt)