Add option to print expected and actual strings with triple quotes

_From @skybrian on May 27, 2016 2:31_

For example, this is not terribly readable:

```
Expected: '///\n'
'// Generated code. Do not modify.\n'
'///\n'
'library test;\n'
'\n'
'import \'dart:async\';\n'
'\n'
'import \'package:protobuf/protobuf.dart\';\n'
'\n'
''
Actual: '///\n'
'// Generated code. Do not modify.\n'
'///\n'
'library test;\n'
'\n'
'import \'dart:async\';\n'
'\n'
'import \'package:protobuf/protobuf.dart\';\n'
'\n'
'class Api {\n'
' RpcClient _client;\n'
' Api(this._client);\n'
'\n'
'}\n'
'\n'
''
Which: is different. Both strings start the same, but the given value also has the following trailing characters: class Api ...
```

Since I use triple-quoted strings for expected data, updating the test is a bit tedious. It would be nicer if there were an option to print diffs using triple-quoted strings. It would also make the output easier to read.

_Copied from original issue: dart-lang/test#435_

Start by tracing the test failure output that produces the shown Expected/Actual string diff; the issue does not name a source file or test entry point. Add an option for diffs to use triple-quoted strings, and verify that long multiline expected and actual values are easier to read.

Written by the indexing model from the issue text.