Fill in coverage gaps for Component2

[greglittlefield-wf]

Motivation

There were missing tests for some of the new Component2 code.

Changes

• Add full test coverage for JsBackedMap methods
  • Fix hashCode in DDC
• Add missing test coverage for setStateWithUpdater arguments
• Add tests for Component2Bridge
• Add tests for throwing UnsupportedError, fix missing throws keywords

Boyscouting:

• Make bridgeForComponent protected.

Testing Instrructions

• Ensure CI passes

[aaronlademann-wf]

The failures are in the dev phase of the build (using Dart 2.6.0-dev)... which has a bug causing the failures: https://github.com/dart-lang/sdk/issues/38311

+10 to integration (CI Passing on Dart 2 stable)