Add strong mode test suite

[sgjesse]

Add a strong mode test suite like the warnings suite (tools/testing/dart/dartino_warnings_suite.dart) we already have. So in addition to

tools/test.py warnings

we get

tools/test.py strong

[sgjesse]

The intend for this test is to ensure that the Dartino related code (samples and packages) shipped in the SDK are strong mode clean.

All the tools including the compiler are not required to be strong mode clean for now.

[mit-mit]

Closed by mistake, reopening.