Implement 'covariant' modifier for dart2js.

pkg/compiler/lib/src/parser/parser.dart

Index: pkg/compiler/lib/src/parser/parser.dart
diff --git a/pkg/compiler/lib/src/parser/parser.dart b/pkg/compiler/lib/src/parser/parser.dart
index 988cbf32c63d165be72ac4250dba5fc1b794c509..40389c560798a4345ce916dfb256c462b6abce4e 100644
--- a/pkg/compiler/lib/src/parser/parser.dart
+++ b/pkg/compiler/lib/src/parser/parser.dart
@@ -426,6 +426,15 @@ class Parser {
   Token parseFormalParameter(Token token, FormalParameterType type) {
     token = parseMetadataStar(token, forParameter: true);
     listener.beginFormalParameter(token);
+
+    // Skip over `covariant` token, if the next token is an identifier or
+    // modifier.
+    // This enables the case where `covariant` is the name of the parameter:
+    //    void foo(covariant);
+    if (identical(token.stringValue, 'covariant') &&
+        token.next.isIdentifier() || isModifier(token.next)) {
+      token = token.next;
+    }
     token = parseModifiers(token);
     // TODO(ahe): Validate that there are formal parameters if void.
     token = parseReturnTypeOpt(token);
@@ -988,9 +997,29 @@ class Parser {
     return null;
   }
 
+  /// Removes the optional `covariant` token from the modifiers, if there
+  /// is no `static` in the list, and `covariant` is the first modifier.
+  Link<Token> removeOptCovariantTokenIfNotStatic(Link<Token> modifiers) {
+    if (modifiers.isEmpty ||
+        !identical(modifiers.first.stringValue, 'covariant')) {
+      return modifiers;
+    }
+    for (Token modifier in modifiers.tail) {
+      if (identical(modifier.stringValue, 'static')) {
+        return modifiers;
+      }
+    }
+    return modifiers.tail;
+  }
+
   Token parseFields(Token start, Link<Token> modifiers, Token type,
       Token getOrSet, Token name, bool isTopLevel) {
     bool hasType = type != null;
+
+    if (getOrSet == null && !isTopLevel) {
+      modifiers = removeOptCovariantTokenIfNotStatic(modifiers);
+    }
+
     Token varFinalOrConst =
         expectVarFinalOrConst(modifiers, hasType, !isTopLevel);
     bool isVar = false;