Enable strong mode in DDC, fix all warnings/errors

lib/src/compiler/js_metalet.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): import from its own package
 import '../js_ast/js_ast.dart';
 import '../js_ast/precedence.dart';
 
 import 'js_names.dart' show TemporaryId;
 
@@ skipping 135 matching lines @@
   int get precedenceLevel => variables.isEmpty ? EXPRESSION : CALL;
 
   /// Patch to pretend [Template] supports visitMetaLet.
   Instantiator _templateVisitMetaLet(InstantiatorGeneratorVisitor visitor) {
     var valueInstantiators = variables.values.map(visitor.visit);
     var bodyInstantiators = body.map(visitor.visit);
 
     return (args) => new MetaLet(
         new Map.fromIterables(
             variables.keys, valueInstantiators.map((i) => i(args))),
-        bodyInstantiators.map((i) => i(args)).toList(),
+        bodyInstantiators.map((i) => i(args) as Expression).toList(),
         statelessResult: statelessResult);
   }
 
   Expression _toInvokedFunction(Statement block) {
     var finder = new _YieldFinder();
     block.accept(finder);
     if (!finder.hasYield) {
       return new Call(new ArrowFun([], block), []);
     }
     // If we have a yield, it's more tricky. We'll create a `function*`, which
@@ skipping 81 matching lines @@
     if (isDeclaration) {
       // Technically, putting one of these in a comma expression is not
       // legal. However when isDeclaration is true, toStatement will be
       // called immediately on the MetaLet, which results in legal JS.
       assign = new VariableDeclarationList(
           'let', [new VariableInitialization(left, value)]);
     } else {
       assign = value.toAssignExpression(left);
     }
 
-    var newBody = new Expression.binary([assign]..addAll(body), ',');
+    assert(body.isNotEmpty);
+    Binary newBody = new Expression.binary([assign]..addAll(body), ',');
     newBody = _substitute(newBody, {result: left});
     return new MetaLet(vars, newBody.commaToExpressionList(),
         statelessResult: statelessResult);
   }
 }
 
 /// Similar to [Template.instantiate] but works with free variables.
 Node _substitute(Node tree, Map<MetaLetVariable, Expression> substitutions) {
   var generator = new InstantiatorGeneratorVisitor(/*forceCopy:*/ false);
   var instantiator = generator.compile(tree);
@@ skipping 79 matching lines @@
   @override
   visitYield(Yield node) {
     if (!_nestedFunction) hasYield = true;
   }
 
   @override
   visitNode(Node node) {
     if (!hasYield) super.visitNode(node);
   }
 }