Update dart2js parser to disallow repeated comparisons and prefix +.

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index 75f36f38c129567970475d49007a8482c4cb69a0..f88863ca6159fcb4e045f3e030d7f3bf8be0985b 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -815,7 +815,7 @@ main() => foo(1);
                 "removing the 'const' modifier.",
       examples: const ["""
 void main() {
-  const c; // This constant variable must be initialized. 
+  const c; // This constant variable must be initialized.
 }"""]);
 
   static const MessageKind MEMBER_USES_CLASS_NAME = const MessageKind(
@@ -915,6 +915,11 @@ Error: "#{value}" is not a valid Symbol name because is not:
       'Error: "!==" is not an operator. '
       'Did you mean "#{lhs} != #{rhs}" or "!identical(#{lhs}, #{rhs})"?');
 
+  static const MessageKind UNSUPPORTED_PREFIX_PLUS = const MessageKind(
+      "Error: '+' is not a prefix operator. ",
+      howToFix: "Try removing '+'.",
+      examples: const [ "+2  // No longer a valid way to write '2'" ]);
+
   static const MessageKind UNSUPPORTED_THROW_WITHOUT_EXP = const MessageKind(
       'Error: No expression after "throw". '
       'Did you mean "rethrow"?');