Use short array syntax in method signatures

test/codegen/expect/cascade.js

 var cascade = dart.defineLibrary(cascade, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class A extends core.Object {
     A() {
       this.x = null;
     }
   }
   dart.setSignature(A, {});
@@ skipping 88 matching lines @@
       super.Base();
     }
     test_final_field_generic(t) {
       this.x[core.$add](1);
       this.x[core.$add](2);
       this.x[core.$add](3);
       this.x[core.$add](4);
     }
   }
   dart.setSignature(Foo, {
-    methods: () => ({test_final_field_generic: dart.functionType(dart.void, [dart.dynamic])})
+    methods: () => ({test_final_field_generic: [dart.void, [dart.dynamic]]})
   });
   // Exports:
   exports.A = A;
   exports.test_closure_with_mutate = test_closure_with_mutate;
   exports.test_closure_without_mutate = test_closure_without_mutate;
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
   exports.test_increment = test_increment;
   exports.Base$ = Base$;
   exports.Base = Base;
   exports.Foo = Foo;
 })(cascade, core);