Add language tests for Issue 18628.

tests/README

+Run Existing Tests
+==================
+
+See the output of
+
+  ../tools/test.py --help
+
+for how to run tests.
+
+See also
+
+  https://code.google.com/p/dart/wiki/Building#Testing
+
+for detailed examples.
+
+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.