Enable strong mode in DDC, fix all warnings/errors

lib/src/js_ast/type_printer.dart

 // Copyright (c) 2016, 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 js_ast;
 
 abstract class _TypePrinterBase implements TypeRefVisitor {
   void out(String s);
   void visit(Node node);
 
@@ skipping 130 matching lines @@
 
   @override
   visitIdentifier(Identifier node) {
     //out(localNamer.getName(node));
     out(node.name);
   }
 
   @override
   visitAccess(PropertyAccess node) {
     var selector = node.selector;
-    assert(selector is LiteralString);
-    if (selector is! LiteralString) {
+    if (selector is LiteralString) {
+      visit(node.receiver);
+      out(".");
+      out(selector.valueWithoutQuotes);
+    } else {
+      assert(false);
       out("?");
-      return;
     }
-    visit(node.receiver);
-    out(".");
-    out(selector.valueWithoutQuotes);
   }
 
   @override toString() => _buffer.toString();
 
   @override
   visitArrayTypeRef(ArrayTypeRef node) {
     out("Array");
     if (node.elementType != null) {
       out("<");
       visit(node.elementType);
@@ skipping 48 matching lines @@
         outSeparated(", ", node.paramTypes.values);
       }
       out(')');
       if (node.returnType != null) {
         out(":");
         visit(node.returnType);
       }
     }
   }
 }