allow JS builtin to be typed as needed

lib/runtime/dart/_interceptors.js

 dart_library.library('dart/_interceptors', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/_internal',
   'dart/collection',
   'dart/math'
 ], /* Lazy imports */[
   'dart/_js_helper'
 ], function(exports, dart, core, _internal, collection, math, _js_helper) {
   'use strict';
@@ skipping 105 matching lines @@
         this[dartx.checkGrowable]('insertAll');
         _internal.IterableMixinWorkaround.insertAllList(this, index, iterable);
       }
       [dartx.setAll](index, iterable) {
         dart.as(iterable, core.Iterable$(E));
         _internal.IterableMixinWorkaround.setAllList(this, index, iterable);
       }
       [dartx.removeLast]() {
         this[dartx.checkGrowable]('removeLast');
         if (this[dartx.length] == 0) dart.throw(new core.RangeError.value(-1));
-        return dart.as(this.pop(), E);
+        return this.pop();
       }
       [dartx.remove](element) {
         this[dartx.checkGrowable]('remove');
         for (let i = 0; dart.notNull(i) < dart.notNull(this[dartx.length]); i = dart.notNull(i) + 1) {
           if (dart.equals(this[dartx.get](i), element)) {
             this.splice(i, 1);
             return true;
           }
         }
         return false;
@@ skipping 20 matching lines @@
           this[dartx.add](e);
         }
       }
       [dartx.clear]() {
         this[dartx.length] = 0;
       }
       [dartx.forEach](f) {
         dart.as(f, dart.functionType(dart.void, [E]));
         let length = this[dartx.length];
         for (let i = 0; dart.notNull(i) < dart.notNull(length); i = dart.notNull(i) + 1) {
-          f(dart.as(this[i], E));
+          f(this[i]);
           if (length != this[dartx.length]) {
             dart.throw(new core.ConcurrentModificationError(this));
           }
         }
       }
       [dartx.map](f) {
         dart.as(f, dart.functionType(dart.dynamic, [E]));
         return _internal.IterableMixinWorkaround.mapList(this, f);
       }
       [dartx.join](separator) {
@@ skipping 168 matching lines @@
       }
       set [dartx.length](newLength) {
         if (!(typeof newLength == 'number')) dart.throw(new core.ArgumentError(newLength));
         if (dart.notNull(newLength) < 0) dart.throw(new core.RangeError.value(newLength));
         this[dartx.checkGrowable]('set length');
         this.length = newLength;
       }
       [dartx.get](index) {
         if (!(typeof index == 'number')) dart.throw(new core.ArgumentError(index));
         if (dart.notNull(index) >= dart.notNull(this[dartx.length]) || dart.notNull(index) < 0) dart.throw(new core.RangeError.value(index));
-        return dart.as(this[index], E);
+        return this[index];
       }
       [dartx.set](index, value) {
         dart.as(value, E);
         if (!(typeof index == 'number')) dart.throw(new core.ArgumentError(index));
         if (dart.notNull(index) >= dart.notNull(this[dartx.length]) || dart.notNull(index) < 0) dart.throw(new core.RangeError.value(index));
         this[index] = value;
         return value;
       }
       [dartx.asMap]() {
         return new (_internal.IterableMixinWorkaround$(E))().asMapList(this);
@@ skipping 601 matching lines @@
       _js_helper.checkString(to);
       _js_helper.checkInt(startIndex);
       if (dart.notNull(startIndex) < 0 || dart.notNull(startIndex) > dart.notNull(this[dartx.length])) {
         dart.throw(new core.RangeError.range(startIndex, 0, this[dartx.length]));
       }
       return dart.as(_js_helper.stringReplaceFirstUnchecked(this, from, to, startIndex), core.String);
     }
     [dartx.split](pattern) {
       _js_helper.checkNull(pattern);
       if (typeof pattern == 'string') {
-        return dart.as(this.split(pattern), core.List$(core.String));
+        return this.split(pattern);
       } else if (dart.is(pattern, _js_helper.JSSyntaxRegExp) && _js_helper.regExpCaptureCount(pattern) == 0) {
         let re = _js_helper.regExpGetNative(pattern);
-        return dart.as(this.split(re), core.List$(core.String));
+        return this.split(re);
       } else {
         return this[_defaultSplit](pattern);
       }
     }
     [_defaultSplit](pattern) {
       let result = dart.list([], core.String);
       let start = 0;
       let length = 1;
       for (let match of pattern[dartx.allMatches](this)) {
         let matchStart = match.start;
@@ skipping 429 matching lines @@
   exports.JSString = JSString;
   exports.getInterceptor = getInterceptor;
   exports.JSBool = JSBool;
   exports.JSIndexable = JSIndexable;
   exports.JSMutableIndexable = JSMutableIndexable;
   exports.JSObject = JSObject;
   exports.JavaScriptObject = JavaScriptObject;
   exports.PlainJavaScriptObject = PlainJavaScriptObject;
   exports.UnknownJavaScriptObject = UnknownJavaScriptObject;
 });