Enable strong mode in DDC, fix all warnings/errors

lib/src/js_ast/builder.dart

Index: lib/src/js_ast/builder.dart
diff --git a/lib/src/js_ast/builder.dart b/lib/src/js_ast/builder.dart
index 5037a3f0ef142003125f4e83838acf2d5f2f8dba..8b5f58a3d63d643bf9bb88514d555afa4594aad3 100644
--- a/lib/src/js_ast/builder.dart
+++ b/lib/src/js_ast/builder.dart
@@ -1154,7 +1154,7 @@ class MiniJsParser {
   /** Parse a variable declaration list, with `var` or `let` [keyword] */
   VariableDeclarationList parseVariableDeclarationList(
       String keyword, [String firstIdentifier])  {
-    var initialization = [];
+    var initialization = <VariableInitialization>[];
 
     do {
       var declarator;
@@ -1514,7 +1514,7 @@ class MiniJsParser {
       }
       expectCategory(COLON);
     }
-    List statements = new List<Statement>();
+    var statements = <Statement>[];
     while (lastCategory != RBRACE &&
            lastToken != 'case' &&
            lastToken != 'default') {