Add a few more known non-null cases

lib/runtime/dart/_internal.js

 dart_library.library('dart/_internal', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/collection'
 ], /* Lazy imports */[
   'dart/math',
   'dart/_interceptors',
   'dart/_js_primitives'
 ], function(exports, dart, core, collection, math, _interceptors, _js_primitives) {
   'use strict';
@@ skipping 126 matching lines @@
             dart.throw(new core.ConcurrentModificationError(this));
           }
           let buffer = new core.StringBuffer(first);
           for (let i = 1; i < dart.notNull(length); i++) {
             buffer.write(separator);
             buffer.write(this.elementAt(i));
             if (length != this.length) {
               dart.throw(new core.ConcurrentModificationError(this));
             }
           }
-          return dart.toString(buffer);
+          return buffer.toString();
         } else {
           let buffer = new core.StringBuffer();
           for (let i = 0; i < dart.notNull(length); i++) {
             buffer.write(this.elementAt(i));
             if (length != this.length) {
               dart.throw(new core.ConcurrentModificationError(this));
             }
           }
-          return dart.toString(buffer);
+          return buffer.toString();
         }
       }
       where(test) {
         dart.as(test, dart.functionType(core.bool, [E]));
         return super.where(test);
       }
       map(f) {
         dart.as(f, dart.functionType(dart.dynamic, [E]));
         return new (MappedListIterable$(E, dart.dynamic))(this, f);
       }
@@ skipping 1055 matching lines @@
         for (let element of iterable) {
           if (index == elementIndex) return element;
           elementIndex++;
         }
         dart.throw(core.RangeError.index(index, iterable, "index", null, elementIndex));
       }
       static join(iterable, separator) {
         if (separator === void 0) separator = null;
         let buffer = new core.StringBuffer();
         buffer.writeAll(iterable, separator);
-        return dart.toString(buffer);
+        return buffer.toString();
       }
       static joinList(list, separator) {
         if (separator === void 0) separator = null;
         if (dart.notNull(list[dartx.isEmpty])) return "";
         if (list[dartx.length] == 1) return `${list[dartx.get](0)}`;
         let buffer = new core.StringBuffer();
         if (dart.notNull(separator[dartx.isEmpty])) {
           for (let i = 0; i < dart.notNull(list[dartx.length]); i++) {
             buffer.write(list[dartx.get](i));
           }
         } else {
           buffer.write(list[dartx.get](0));
           for (let i = 1; i < dart.notNull(list[dartx.length]); i++) {
             buffer.write(separator);
             buffer.write(list[dartx.get](i));
           }
         }
-        return dart.toString(buffer);
+        return buffer.toString();
       }
       where(iterable, f) {
         dart.as(iterable, core.Iterable$(T));
         dart.as(f, dart.functionType(core.bool, [T]));
         return new (WhereIterable$(T))(iterable, f);
       }
       static map(iterable, f) {
         return MappedIterable.new(iterable, f);
       }
       static mapList(list, f) {
@@ skipping 661 matching lines @@
         for (let i = dart.notNull(srcStart) + dart.notNull(count) - 1, j = dart.notNull(dstStart) + dart.notNull(count) - 1; i >= dart.notNull(srcStart); i--, j--) {
           dst[dartx.set](j, src[dartx.get](i));
         }
       } else {
         for (let i = srcStart, j = dstStart; dart.notNull(i) < dart.notNull(srcStart) + dart.notNull(count); i = dart.notNull(i) + 1, j = dart.notNull(j) + 1) {
           dst[dartx.set](j, src[dartx.get](i));
         }
       }
     }
     static areEqual(a, b) {
-      if (dart.notNull(core.identical(a, b))) return true;
+      if (core.identical(a, b)) return true;
       if (!dart.is(b, core.List)) return false;
       let length = a[dartx.length];
       if (!dart.equals(length, dart.dload(b, 'length'))) return false;
       for (let i = 0; i < dart.notNull(length); i++) {
-        if (!dart.notNull(core.identical(a[dartx.get](i), dart.dindex(b, i)))) return false;
+        if (!core.identical(a[dartx.get](i), dart.dindex(b, i))) return false;
       }
       return true;
     }
     static indexOf(a, element, startIndex, endIndex) {
       if (dart.notNull(startIndex) >= dart.notNull(a[dartx.length])) {
         return -1;
       }
       if (dart.notNull(startIndex) < 0) {
         startIndex = 0;
       }
@@ skipping 412 matching lines @@
   exports.ReversedListIterable = ReversedListIterable;
   exports.UnmodifiableListError = UnmodifiableListError;
   exports.NonGrowableListError = NonGrowableListError;
   exports.makeListFixedLength = makeListFixedLength;
   exports.Lists = Lists;
   exports.printToConsole = printToConsole;
   exports.Sort = Sort;
   exports.Symbol = Symbol;
   exports.POWERS_OF_TEN = POWERS_OF_TEN;
 });