Add language tests for Issue 18628.

tests/README

+Run Existing Tests
+==================
+
+See the output of
+
+  ../tools/test.py --help
+
+for how to run tests.
+
+Also:
+- https://code.google.com/p/dart/wiki/TestingDart2js
+- https://code.google.com/p/dart/wiki/Building#Testing
+
+Examples
+--------
+
+Build everything once before testing:
+
+  ../tools/build.py --mode release --arch x64
+
+Run a subset of the dartanalyzer tests:

[karlklose, 2014/09/01 09:08:18]

I think these examples are to specific for this file.

In general, I think we should add examples to the wiki pages and only add a
pointer to these pages here.

+
+  ../tools/test.py \
+    --compiler dartanalyzer \
+    --runtime none \
+    --progress color \
+    --arch x64 \
+    --mode release \
+    --host-checked \

[karlklose, 2014/09/01 09:08:18]

host-checked is only important for dart2js, where it is so slow that we usually
do not run it unless we changed the compiler itself.

+    --checked \
+    --report --time \
+    --tasks 6 \
+    language/issue18628

[karlklose, 2014/09/01 09:08:18]

'language/issue18628' -> 'language'?

But, as said above, I would remove the examples.

+
+Run a subset of the vm tests in checked mode:
+
+  ../tools/test.py \
+    --compiler none \
+    --runtime vm \
+    --progress color \
+    --arch x64 \
+    --mode release \
+    --host-checked \
+    --checked \
+    --report --time \
+    --tasks 6 \
+    language/issue18628
+
+Run a subset of the dart2js tests in checked mode:
+
+  ../tools/test.py \
+    --compiler dart2js \
+    --runtime chrome \
+    --progress color \
+    --arch x64 \
+    --mode release \
+    --host-checked \
+    --checked \
+    --report --time \
+    --tasks 6 \
+    language/issue18628
+
+Create New Tests
+================
+
+See comments above
+
+  factory StandardTestSuite.forDirectory
+
+in
+
+  ../tools/testing/dart/test_suite.dart
+
+for the default test directory layout. By default test-file names must
+end in "_test.dart", but some test suites, such as ./co19, subclass
+StandardTestSuite and override this default.
+
+See comments at the beginning of
+
+  ../tools/testing/dart/multitest.dart
+
+for how to create tests that pass by failing with a known error. For
+example,
+
+  ...
+  int x = "not an int"; /// 01: static type warning
+  ...
+
+as part of a test will only pass the "--compiler dartanalyzer" test if
+the assignment generates a static type warning.