Improve parser error recovery.

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

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 1820 matching lines @@
 ''']);
 
   static const MessageKind UNTERMINATED_COMMENT = const MessageKind(
       "Comment starting with '/*' must end with '*/'.",
       howToFix: DONT_KNOW_HOW_TO_FIX,
       examples: const [r"""
 main() {
 }
 /*"""]);
 
+  static const MessageKind MISSING_TOKEN_BEFORE_THIS = const MessageKind(
+      "Expected '#{token}' before this.",
+      // Consider the second example below: the parser expects a ')' before
+      // 'y', but a ',' would also have worked. We don't have enough
+      // information to give a good suggestion.
+      howToFix: DONT_KNOW_HOW_TO_FIX,
+      examples: const [
+          "main() => true ? 1;",
+          "main() => foo(x: 1 y: 2);",
+        ]);
+
+  static const MessageKind MISSING_TOKEN_AFTER_THIS = const MessageKind(
+      "Expected '#{token}' after this.",
+      // See [MISSING_TOKEN_BEFORE_THIS], we don't have enough information to
+      // give a good suggestion.
+      howToFix: DONT_KNOW_HOW_TO_FIX,
+      examples: const ["main(x) {x}"]);
+
   static const MessageKind COMPILER_CRASHED = const MessageKind(
       "The compiler crashed when compiling this element.");
 
   static const MessageKind PLEASE_REPORT_THE_CRASH = const MessageKind('''
 The compiler is broken.
 
 When compiling the above element, the compiler crashed. It is not
 possible to tell if this is caused by a problem in your program or
 not. Regardless, the compiler should not crash.
 
@@ skipping 174 matching lines @@
   String message;
 
   Message(this.kind, this.arguments, this.terse) {
     assert(() { computeMessage(); return true; });
   }
 
   String computeMessage() {
     if (message == null) {
       message = kind.template;
       arguments.forEach((key, value) {
-        message = message.replaceAll('#{${key}}', value.toString());
+        message = message.replaceAll('#{${key}}', convertToString(value));
       });
       assert(invariant(
           CURRENT_ELEMENT_SPANNABLE,
           kind == MessageKind.GENERIC ||
             !message.contains(new RegExp(r'#\{.+\}')),
           message: 'Missing arguments in error message: "$message"'));
       if (!terse && kind.hasHowToFix) {
         String howToFix = kind.howToFix;
         arguments.forEach((key, value) {
-          howToFix = howToFix.replaceAll('#{${key}}', value.toString());
+          howToFix = howToFix.replaceAll('#{${key}}', convertToString(value));
         });
         message = '$message\n$howToFix';
       }
     }
     return message;
   }
 
   String toString() {
     return computeMessage();
   }
 
   bool operator==(other) {
     if (other is !Message) return false;
     return (kind == other.kind) && (toString() == other.toString());
   }
 
   int get hashCode => throw new UnsupportedError('Message.hashCode');
+
+  static String convertToString(value) {
+    if (value is ErrorToken) {
+      // Shouldn't happen.
+      return value.assertionMessage;
+    } else if (value is Token) {
+      value = value.value;
+    }
+    return '$value';
+  }
 }