Make it easy/efficient to create immutable collections via literals

See

https://github.com/dart-lang/sdk/blob/031e77eea11a16a5e486a3673549adbd68862c8e/sdk/lib/core/uri.dart#L2233-L2237

```dart
// ASCII, a single percent encoded sequence.
codeUnits = new List(3);
codeUnits[0] = _PERCENT;
codeUnits[1] = _hexDigits.codeUnitAt(char >> 4);
codeUnits[2] = _hexDigits.codeUnitAt(char & 0xf);
```

Also in the SDK,
2x in this file https://github.com/dart-lang/sdk/blob/031e77eea11a16a5e486a3673549adbd68862c8e/sdk/lib/vmservice/message.dart#L176-L182

4x in this file https://github.com/dart-lang/sdk/blob/031e77eea11a16a5e486a3673549adbd68862c8e/runtime/bin/vmservice/loader.dart#L1107-L1112

This entire file in `pkg:isolate` - https://github.com/dart-lang/isolate/blob/master/lib/src/util.dart

In the Dart sources in Google (some of these duplicate the above)

- `List.unmodifiable\(\[` - 135 hits
- `List\<.+\>\.unmodifiable\(\[` - 96 times
- `Map\<.+,.+>\.unmodifiable\(\{` - 25 times
- `Map\.unmodifiable\(\{` - 28 times
- `UnmodifiableSetView` from `pkg:collection` is used dozens of times, not as a view over mutable data, but as a immutable set.

Benefits:

- Less copying!
- Immutable collections are more efficient
- iteration doesn't need to track `version`, etc
- no need to have indirection to a fixed-sized backing store to support growing
- Enables special-casing implementations - e.g. `immutable([1,2,3])` could be created as `Uint8List`.

Start by reviewing the linked SDK examples and the collection usage counts in the issue, including sdk/lib/core/uri.dart, sdk/lib/vmservice/message.dart, runtime/bin/vmservice/loader.dart, and pkg:isolate/lib/src/util.dart. Determine the literal syntax, semantics, and implementation scope needed; the work is done when the language proposal resolves these design questions and identifies the affected collection behavior.

Written by the indexing model from the issue text.