Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/tests/standalone/standalone-leg.status

Index: dart/tests/standalone/standalone-leg.status
diff --git a/dart/tests/standalone/standalone-leg.status b/dart/tests/standalone/standalone-leg.status
index 4e2d0b01cb46fd5fd3976da983e125101b5893c3..bff9724e6cce053a71a611ef35360b00127d34f6 100644
--- a/dart/tests/standalone/standalone-leg.status
+++ b/dart/tests/standalone/standalone-leg.status
@@ -3,12 +3,15 @@
 # BSD-style license that can be found in the LICENSE file.
 
 [ $component == dart2js ]
-AssertTest: Fail, OK # Do not crash.
-ByteArrayTest: Fail, OK # Do not crash.
-DeoptimizationTest: Fail, OK # Do not crash.
-io/*: Fail, OK # Do not crash.
-io/HttpDateTest: Pass
-io/ProcessExitNegativeTest: Pass
-io/UrlEncodingTest: Pass
-MediumIntegerTest: Fail, OK # Do not crash.
-OutOfMemoryTest: Fail, OK # Do not crash.
+AssertTest: Fail # Assert not implemented
+
+
+DeoptimizationTest: Fail, OK # Requires bigint.
+io/DirectoryInvalidArgumentsTest: Fail, OK # Broken test: DirectoryException does not exist
+io/HttpParserTest: Fail, OK # ByteArray
+io/OptionsTest: Fail, OK # Cannot pass options to d8.
+io/TestExtensionTest: Fail, OK # dart-ext:test_extension: Illegal argument(s): dart-ext:test_extension
+io/TestRunnerTest: Fail, OK # library not found dart:builtin
+ByteArrayTest: Fail, OK # ByteArray
+MediumIntegerTest: Fail, OK # cannot resolve type Mint
+OutOfMemoryTest: Fail, OK # d8 handles much larger arrays than Dart VM.