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

test/splitting/constructors.unit

 40 columns                              |
 >>> Single initializers can be on one line
 class Foo extends Bar {
   final int b;
   Foo(int a, this.b) : super(a);
 }
 <<<
 class Foo extends Bar {
   final int b;
   Foo(int a, this.b) : super(a);
@@ skipping 29 matching lines @@
     new Thing(argument, argument);
 >>> splits before ":" if the parameter list does not fit on one line
 class Foo {
   Foo(int longArg1, int longArg2, int longArg3) : this._(longArg1);
 }
 <<<
 class Foo {
   Foo(int longArg1, int longArg2,
       int longArg3)
       : this._(longArg1);
-}
+}
+>>> indent parameters more if body is a wrapped =>
+class Foo {
+  Foo(firstArgument, secondArgument, third) => "very long body that must wrap";
+}
+<<<
+class Foo {
+  Foo(firstArgument, secondArgument,
+          third) =>
+      "very long body that must wrap";
+}