Add a few more known non-null cases

lib/runtime/dart/_isolate_helper.js

Index: lib/runtime/dart/_isolate_helper.js
diff --git a/lib/runtime/dart/_isolate_helper.js b/lib/runtime/dart/_isolate_helper.js
index eec8f7a851eaff9b511e6d589b16b5488b1a1bd8..af6ce9149f83be2cd711fd176cc491a1e84de98e 100644
--- a/lib/runtime/dart/_isolate_helper.js
+++ b/lib/runtime/dart/_isolate_helper.js
@@ -621,7 +621,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
     }
     handleUncaughtError(error, stackTrace) {
       if (dart.notNull(this.errorPorts.isEmpty)) {
-        if (dart.notNull(this.errorsAreFatal) && dart.notNull(core.identical(this, exports._globalState.rootContext))) {
+        if (dart.notNull(this.errorsAreFatal) && core.identical(this, exports._globalState.rootContext)) {
           return;
         }
         if (self.console && self.console.error) {
@@ -651,7 +651,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
         this.handleUncaughtError(e, s);
         if (dart.notNull(this.errorsAreFatal)) {
           this.kill();
-          if (dart.notNull(core.identical(this, exports._globalState.rootContext))) {
+          if (core.identical(this, exports._globalState.rootContext)) {
             throw e;
           }
         }
@@ -1487,7 +1487,7 @@ dart_library.library('dart/_isolate_helper', null, /* Imports */[
       return hash;
     }
     ['=='](other) {
-      if (dart.notNull(core.identical(other, this))) return true;
+      if (core.identical(other, this)) return true;
       if (dart.is(other, CapabilityImpl)) {
         return core.identical(this[_id], other[_id]);
       }