Implement Symbol correctly in dart2js.

dart/sdk/lib/_collection_dev/symbol.dart

Index: dart/sdk/lib/_collection_dev/symbol.dart
diff --git a/dart/sdk/lib/_collection_dev/symbol.dart b/dart/sdk/lib/_collection_dev/symbol.dart
index 1a81f4767fd1c08d18f34cc543d27bb04ea114f1..7fd79d3d0e570ebf793bb230a46df41923f1f7bb 100644
--- a/dart/sdk/lib/_collection_dev/symbol.dart
+++ b/dart/sdk/lib/_collection_dev/symbol.dart
@@ -15,6 +15,30 @@ part of dart._collection.dev;
 class Symbol implements core.Symbol {
   final String _name;
 
+  static final RegExp validationPattern =
+      new RegExp(r'^(?:[a-zA-Z$][a-zA-Z$0-9_]*\.)*(?:[a-zA-Z$][a-zA-Z$0-9_]*=?|'
+                 r'-|'
+                 r'unary-|'
+                 r'\[\]=|'
+                 r'~|'
+                 r'==|'
+                 r'\[\]|'
+                 r'\*|'
+                 r'/|'
+                 r'%|'
+                 r'~/|'
+                 r'\+|'
+                 r'<<|'
+                 r'>>|'
+                 r'>=|'
+                 r'>|'
+                 r'<=|'
+                 r'<|'
+                 r'&|'
+                 r'\^|'
+                 r'\|'
+                 r')$');
+
   external const Symbol(String name);
 
   /**
@@ -23,6 +47,10 @@ class Symbol implements core.Symbol {
    */
   const Symbol.unvalidated(this._name);
 
+  // This is called by dart2js.
+  Symbol.validated(String name)
+      : this._name = validate(name);
+
   bool operator ==(other) => other is Symbol && _name == other._name;
 
   int get hashCode {
@@ -32,4 +60,16 @@ class Symbol implements core.Symbol {
 
   /// Platform-private accessor which cannot be called from user libraries.
   static String getName(Symbol symbol) => symbol._name;
+
+  static String validate(String name) {
+    if (name.isEmpty) return name;
+    if (name.startsWith('_')) {
+      throw new ArgumentError('"$name" is a private identifier');
+    }
+    if (!validationPattern.hasMatch(name)) {
+      throw new ArgumentError(
+          '"$name" is not an identifier or an empty String');
+    }
+    return name;
+  }
 }