Enable strong mode in DDC, fix all warnings/errors

lib/src/compiler/js_metalet.dart

Index: lib/src/compiler/js_metalet.dart
diff --git a/lib/src/compiler/js_metalet.dart b/lib/src/compiler/js_metalet.dart
index c0581d638a1ed6f1ac10fdcff68c98b7c63f954c..8e341fe5e341d6602dfd73da10405117002a7bb3 100644
--- a/lib/src/compiler/js_metalet.dart
+++ b/lib/src/compiler/js_metalet.dart
@@ -153,7 +153,7 @@ class MetaLet extends Expression {
     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);
   }
 
@@ -255,7 +255,8 @@ class MetaLet extends Expression {
       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);