Add command-line parameter enabling assert messages.

pkg/compiler/lib/src/patch_parser.dart

Index: pkg/compiler/lib/src/patch_parser.dart
diff --git a/pkg/compiler/lib/src/patch_parser.dart b/pkg/compiler/lib/src/patch_parser.dart
index 0631b16bed903eb01a281b8d267a8a11ce7987b6..70bb3bf615de816589552418ad35e49ba54a3807 100644
--- a/pkg/compiler/lib/src/patch_parser.dart
+++ b/pkg/compiler/lib/src/patch_parser.dart
@@ -177,7 +177,9 @@ class PatchParserTask extends CompilerTask {
                                                         compilationUnit,
                                                         idGenerator);
       try {
-        new PartialParser(patchListener).parseUnit(tokens);
+        new PartialParser(patchListener,
+                          allowAssertMessage: compiler.allowAssertMessage)
+            .parseUnit(tokens);
       } on ParserError catch (e) {
         // No need to recover from a parser error in platform libraries, user
         // will never see this if the libraries are tested correctly.
@@ -194,7 +196,8 @@ class PatchParserTask extends CompilerTask {
 
     measure(() => compiler.withCurrentElement(cls, () {
       MemberListener listener = new PatchMemberListener(compiler, cls);
-      Parser parser = new PatchClassElementParser(listener);
+      Parser parser = new PatchClassElementParser(listener,
+          allowAssertMessage: compiler.allowAssertMessage);
       try {
         Token token = parser.parseTopLevelDeclaration(cls.beginToken);
         assert(identical(token, cls.endToken.next));
@@ -241,7 +244,9 @@ class PatchMemberListener extends MemberListener {
  * declarations.
  */
 class PatchClassElementParser extends PartialParser {
-  PatchClassElementParser(Listener listener) : super(listener);
+  PatchClassElementParser(Listener listener,
+                          {bool allowAssertMessage: false})
+      : super(listener, allowAssertMessage: allowAssertMessage);
 
   Token parseClassBody(Token token) => fullParseClassBody(token);
 }