Add a few more known non-null cases

test/codegen/expect/notnull.js

 dart_library.library('notnull', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   function intAssignments() {
     let i = 0;
     i = i & 1;
@@ skipping 206 matching lines @@
 
     try {
       dart.dsend(null, 'foo');
     } catch (e) {
       let trace = dart.stackTrace(e);
       core.print(`${typeof e == 'string' ? e : dart.toString(e)} at ${trace}`);
     }
 
   }
   dart.fn(controlFlow);
+  function cascadesOnNull() {
+    let x = null;
+    core.print(dart.hashCode(((() => {
+      dart.toString(x);
+      dart.runtimeType(x);
+      return x;
+    })())));
+    let y = null;
+    core.print(dart.hashCode(((() => {
+      dart.toString(y);
+      dart.runtimeType(y);
+      return y;
+    })())));
+  }
+  dart.fn(cascadesOnNull);
   function main() {
     intAssignments();
     doubleAssignments();
     boolAssignments();
     nullableLocals(1);
     optParams(1, 2);
     namedParams({x: 1, y: 2});
     forLoops(dart.fn(() => 10, core.int, []));
     increments();
     conditionals(true);
     calls();
     localEscapes();
     controlFlow();
+    cascadesOnNull();
     nullableCycle();
     nonNullableCycle();
   }
   dart.fn(main);
   // Exports:
   exports.intAssignments = intAssignments;
   exports.doubleAssignments = doubleAssignments;
   exports.boolAssignments = boolAssignments;
   exports.increments = increments;
   exports.conditionals = conditionals;
   exports.nullAwareOps = nullAwareOps;
   exports.nullableLocals = nullableLocals;
   exports.optParams = optParams;
   exports.namedParams = namedParams;
   exports.forLoops = forLoops;
   exports.nullableCycle = nullableCycle;
   exports.nonNullableCycle = nonNullableCycle;
   exports.Foo = Foo;
   exports.calls = calls;
   exports.localEscapes = localEscapes;
   exports.controlFlow = controlFlow;
+  exports.cascadesOnNull = cascadesOnNull;
   exports.main = main;
 });