allow JS builtin to be typed as needed

lib/runtime/dart/_js_helper.js

Index: lib/runtime/dart/_js_helper.js
diff --git a/lib/runtime/dart/_js_helper.js b/lib/runtime/dart/_js_helper.js
index b82c3a7c21008af227978e537ff031bee499e715..57beafa89a901ac511ddbdeb02921ece96a825eb 100644
--- a/lib/runtime/dart/_js_helper.js
+++ b/lib/runtime/dart/_js_helper.js
@@ -119,7 +119,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
       dart.throw(new core.FormatException(`Illegal RegExp pattern: ${source}, ${errorMessage}`));
     }
     firstMatch(string) {
-      let m = dart.as(this[_nativeRegExp].exec(checkString(string)), core.List$(core.String));
+      let m = this[_nativeRegExp].exec(checkString(string));
       if (m == null) return null;
       return new _MatchImplementation(this, m);
     }
@@ -143,14 +143,14 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
     [_execGlobal](string, start) {
       let regexp = this[_nativeGlobalVersion];
       regexp.lastIndex = start;
-      let match = dart.as(regexp.exec(string), core.List);
+      let match = regexp.exec(string);
       if (match == null) return null;
       return new _MatchImplementation(this, dart.as(match, core.List$(core.String)));
     }
     [_execAnchored](string, start) {
       let regexp = this[_nativeAnchoredVersion];
       regexp.lastIndex = start;
-      let match = dart.as(regexp.exec(string), core.List);
+      let match = regexp.exec(string);
       if (match == null) return null;
       if (match[dartx.get](dart.notNull(match[dartx.length]) - 1) != null) return null;
       match[dartx.length] = dart.notNull(match[dartx.length]) - 1;
@@ -496,7 +496,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
   }
   dart.fn(stringJoinUnchecked);
   function getRuntimeType(object) {
-    return dart.as(dart.realRuntimeType(object), core.Type);
+    return dart.realRuntimeType(object);
   }
   dart.fn(getRuntimeType, core.Type, [dart.dynamic]);
   function getIndex(array, index) {
@@ -528,7 +528,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
       Primitives.mirrorInvokeCacheName = dart.notNull(Primitives.mirrorInvokeCacheName) + `_${id}`;
     }
     static objectHashCode(object) {
-      let hash = dart.as(object.$identityHash, core.int);
+      let hash = object.$identityHash;
       if (hash == null) {
         hash = Math.random() * 0x3fffffff | 0;
         object.$identityHash = hash;
@@ -697,11 +697,11 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
     }
     static getTimeZoneName(receiver) {
       let d = Primitives.lazyAsJsDate(receiver);
-      let match = dart.as(/\((.*)\)/.exec(d.toString()), core.List);
+      let match = /\((.*)\)/.exec(d.toString());
       if (match != null) return dart.as(match[dartx.get](1), core.String);
-      match = dart.as(/^[A-Z,a-z]{3}\s[A-Z,a-z]{3}\s\d+\s\d{2}:\d{2}:\d{2}\s([A-Z]{3,5})\s\d{4}$/.exec(d.toString()), core.List);
+      match = /^[A-Z,a-z]{3}\s[A-Z,a-z]{3}\s\d+\s\d{2}:\d{2}:\d{2}\s([A-Z]{3,5})\s\d{4}$/.exec(d.toString());
       if (match != null) return dart.as(match[dartx.get](1), core.String);
-      match = dart.as(/(?:GMT|UTC)[+-]\d{4}/.exec(d.toString()), core.List);
+      match = /(?:GMT|UTC)[+-]\d{4}/.exec(d.toString());
       if (match != null) return dart.as(match[dartx.get](0), core.String);
       return "";
     }
@@ -949,7 +949,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
       if (this[_trace] != null) return this[_trace];
       let trace = null;
       if (typeof this[_exception] === "object") {
-        trace = dart.as(this[_exception].stack, core.String);
+        trace = this[_exception].stack;
       }
       return this[_trace] = trace == null ? '' : trace;
     }
@@ -1101,7 +1101,7 @@ dart_library.library('dart/_js_helper', null, /* Imports */[
       }
       moveNext() {
         let ret = this[_jsIterator].next();
-        this[_current] = dart.as(ret.value, E);
+        this[_current] = ret.value;
         return !ret.done;
       }
     }