Add type annotations to remove warnings and fix a bug.

lib/compiler/implementation/dart_backend/placeholder_collector.dart

 // Copyright (c) 2012, 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.
 
 class LocalPlaceholder {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
   int get hashCode => identifier.hashCode;
   String toString() =>
@@ skipping 350 matching lines @@
         LocalPlaceholder localPlaceholder = new LocalPlaceholder(name);
         currentFunctionScope.localPlaceholders.add(localPlaceholder);
         return localPlaceholder;
       });
     }
 
     getLocalPlaceholder().nodes.add(identifier);
   }
 
   void internalError(String reason, {Node node}) {
-    compiler.cancel(reason: reason, node: node);
+    compiler.cancel(reason, node: node);
   }
 
   void unreachable() { internalError('Unreachable case'); }
 
   visit(Node node) => (node == null) ? null : node.accept(this);
 
   visitNode(Node node) { node.visitChildren(this); }  // We must go deeper.
 
   visitSend(Send send) {
     new SendVisitor(this, treeElements).visitSend(send);
@@ skipping 254 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }