Fix constructor initializer space conventions (dartbug.com/17642).

pkg/analyzer/test/services/data/cu_tests.data

 >>>
 class
 A
 {
 }
 <<<
 class A {
 }
 >>> dartbug.com/15876
 // fisk
@@ skipping 198 matching lines @@
 class A extends B {
 
 }
 <<<
 class A extends B {
 
 }
 >>> Single initializers can be on one line
 class Foo extends Bar {
   final int b;
-  Foo(int a, this.b): super(a);
+  Foo(int a, this.b) : super(a);
 }
 <<<
 class Foo extends Bar {
   final int b;
-  Foo(int a, this.b): super(a);
+  Foo(int a, this.b) : super(a);
 }
 >>> (or not)
 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);
 }
 >>> Multiple initializers are one per line
 class Foo extends Bar {
   final int b;
   Foo(int a, int b) : super(a), this.b = b == null ? 0 : b;
 }
 <<<
 class Foo extends Bar {
   final int b;
   Foo(int a, int b)
       : super(a),
         this.b = b == null ? 0 : b;
 }