Use signature inheritance when method types are unchanged

test/codegen/expect/sunflower/sunflower.js

 var sunflower = dart.defineLibrary(sunflower, {});
 var math = dart.import(math);
 var dom = dart.import(dom);
 var core = dart.import(core);
 (function(exports, math, dom, core) {
   'use strict';
   let ORANGE = "orange";
   let SEED_RADIUS = 2;
   let SCALE_FACTOR = 4;
   let TAU = dart.notNull(math.PI) * 2;
@@ skipping 37 matching lines @@
     exports.notes.textContent = `${exports.seeds} seeds`;
   }
   dart.fn(draw, dart.void, []);
   class Circle extends core.Object {
     Circle(x, y, radius) {
       this.x = x;
       this.y = y;
       this.radius = radius;
     }
   }
-  dart.setSignature(Circle, {});
   class CirclePainter extends core.Object {
     CirclePainter() {
       this.color = ORANGE;
     }
     draw() {
       exports.context.beginPath();
       exports.context.lineWidth = 2;
       exports.context.fillStyle = this.color;
       exports.context.strokeStyle = this.color;
       exports.context.arc(this.x, this.y, this.radius, 0, TAU, false);
       exports.context.fill();
       exports.context.closePath();
       exports.context.stroke();
     }
   }
   CirclePainter[dart.implements] = () => [Circle];
   dart.setSignature(CirclePainter, {
     methods: () => ({draw: dart.functionType(dart.void, [])})
   });
   class SunflowerSeed extends dart.mixin(Circle, CirclePainter) {
     SunflowerSeed(x, y, radius, color) {
       if (color === void 0)
         color = null;
       super.Circle(x, y, radius);
       if (color != null)
         this.color = color;
     }
   }
-  dart.setSignature(SunflowerSeed, {});
   // Exports:
   exports.ORANGE = ORANGE;
   exports.SEED_RADIUS = SEED_RADIUS;
   exports.SCALE_FACTOR = SCALE_FACTOR;
   exports.TAU = TAU;
   exports.MAX_D = MAX_D;
   exports.centerX = centerX;
   exports.centerY = centerY;
   exports.querySelector = querySelector;
   exports.main = main;
   exports.draw = draw;
   exports.Circle = Circle;
   exports.CirclePainter = CirclePainter;
   exports.SunflowerSeed = SunflowerSeed;
 })(sunflower, math, dom, core);