allow JS builtin to be typed as needed

lib/runtime/dart/_interceptors.js

Index: lib/runtime/dart/_interceptors.js
diff --git a/lib/runtime/dart/_interceptors.js b/lib/runtime/dart/_interceptors.js
index 598599d6d17e5d87f5b3097de40749a18ac76611..0cc2c7193ca259f1ce7460e8c9a396ee9845c976 100644
--- a/lib/runtime/dart/_interceptors.js
+++ b/lib/runtime/dart/_interceptors.js
@@ -123,7 +123,7 @@ dart_library.library('dart/_interceptors', null, /* Imports */[
       [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');
@@ -164,7 +164,7 @@ dart_library.library('dart/_interceptors', null, /* Imports */[
         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));
           }
@@ -353,7 +353,7 @@ dart_library.library('dart/_interceptors', null, /* Imports */[
       [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);
@@ -975,10 +975,10 @@ dart_library.library('dart/_interceptors', null, /* Imports */[
     [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);
       }