Indent the parameter list more if the body is a wrapped "=>". Fix #144.

test/splitting/parameters.unit

 40 columns                              |
 >>> long parameters list, this.field
 class Foo {
   Foo(this.first, this.second, this.third, this.fourth);
 }
 <<<
 class Foo {
   Foo(this.first, this.second,
       this.third, this.fourth);
 }
+>>> indent parameters more if body is a wrapped =>
+method(int firstArgument, int argumentTwo) => "very long body that must wrap";
+<<<
+method(int firstArgument,
+        int argumentTwo) =>
+    "very long body that must wrap";