Do not suppress implicit constructors that need a super call.

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) {
+    Bar() {
+      super.Foo();
+    }
     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 = {}));