Add some more validation of things in expect that should not be used.

tools/migration/lib/src/validate.dart

Index: tools/migration/lib/src/validate.dart
diff --git a/tools/migration/lib/src/validate.dart b/tools/migration/lib/src/validate.dart
index 5dbbeaaabd9188537d0d0a2441f0ca7297c73efe..c91547ee8b9e61a2f75e9bffd32da79537c312a2 100644
--- a/tools/migration/lib/src/validate.dart
+++ b/tools/migration/lib/src/validate.dart
@@ -9,6 +9,8 @@ final _checkedPattern = new RegExp(r"\bchecked\b");
 final _abstractErrorPattern =
     new RegExp(r"\bAbstractClassInstantiationError\b");
 final _typeErrorPattern = new RegExp(r"\bTypeError\b");
+final _typeAssertionsEnabledPattern = new RegExp(r"\btypeAssertionsEnabled\b");
+final _checkedModeEnabledPattern = new RegExp(r"\bcheckedModeEnabled\b");
 
 void validateFile(String path, String source, [List<String> todos]) {
   check(Pattern pattern, String noteMessage, String todo) {
@@ -24,4 +26,8 @@ void validateFile(String path, String source, [List<String> todos]) {
       "Remove code that checks for AbstractClassInstantiationError.");
   check(_typeErrorPattern, 'mentions "TypeError"',
       "Ensure code that checks for a TypeError uses 2.0 semantics.");
+  check(_typeAssertionsEnabledPattern, 'mentions "typeAssertionsEnabled"',
+      "Remove checks for typeAssertionsEnabled, they are always enabled in 2.0.");
+  check(_checkedModeEnabledPattern, 'mentions "typeAssertionsEnabled"',
+      "Remove checks for checkedModeEnabled, it is always enabled in 2.0.");
 }