Implement operator-is for interfaces' supertypes and String's supertypes. Also, improve stack trace…

dart/tests/language/language-leg.status

Index: dart/tests/language/language-leg.status
diff --git a/dart/tests/language/language-leg.status b/dart/tests/language/language-leg.status
index 8b3c36dd41039646e525133024fb1234f26dfd57..e0afd2b12359278ee6a6bc14d47042c6585079fb 100644
--- a/dart/tests/language/language-leg.status
+++ b/dart/tests/language/language-leg.status
@@ -155,7 +155,6 @@ InstanceCompoundAssignmentOperatorTest: Fail # Expect.equals(expected: <4>, actu
 InstanceFieldInitializerTest: Fail # Expect.equals(expected: <1>, actual: <null>) fails.
 Instanceof2Test: Fail # Expect.equals(expected: <true>, actual: <false>) fails.
 IntrinsifiedMethodsTest: Fail # Expect.isTrue(false) fails.
-IsOperatorTest: Fail # Expect.equals(expected: <true>, actual: <false>) fails.
 LocalFunction3Test: Fail # Expect.equals(expected: <Closure>, actual: <Instance of 'Closure'>) fails.
 ListLiteral3Test: Fail # Expect.equals(expected: <true>, actual: <false>) fails.
 ListLiteral4Test: Fail # Expect.equals(expected: <110111>, actual: <null>) fails.
@@ -177,10 +176,6 @@ StackTraceTest: Fail # ReferenceError: stacktrace is not defined
 StringTest: Fail # TypeError: Object Hello has no method 'operator$indexSet$2'
 
 
-# Bad code.
-InstanceofTest: Fail # SyntaxError: Unexpected identifier
-
-
 #
 # The following tests are all semantic tests that we can rely on dartc
 # catching for now.