add checks needed for covariant generics, and List<E>

test/codegen/expect/covariance.js

+var covariance;
+(function(exports) {
+  'use strict';
+  let _t = Symbol('_t');
+  let Foo$ = dart.generic(function(T) {
+    class Foo extends core.Object {
+      Foo() {
+        this[_t] = null;
+      }
+      add(t) {
+        dart.as(t, T);
+        this[_t] = t;
+      }
+      forEach(fn) {
+        dart.as(fn, dart.functionType(dart.void, [T]));
+        fn(this[_t]);
+      }
+    }
+    return Foo;
+  });
+  let Foo = Foo$();
+  class Bar extends Foo$(core.int) {
+    add(x) {
+      core.print(`Bar.add got ${x}`);
+      super.add(x);
+    }
+  }
+  // Function main: () → dynamic
+  function main() {
+    let foo = new Bar();
+    foo.add('hi');
+  }
+  // Exports:
+  exports.Foo$ = Foo$;
+  exports.Foo = Foo;
+  exports.Bar = Bar;
+  exports.main = main;
+})(covariance || (covariance = {}));