NamedParameters and anonymous classes

Take the following code:

```java
class Foo {
Foo(Demo common) {}
}

class Demo {
Foo bar() {
return new Foo(this) {
// A comment anywhere in the anonymous class
// body that randomly happens to use the name
// of one of the constructor parameters, in this
// case `common`, results in a NamedParameters error.
};
}
}
```

Running error-prone 2.1.0 results in:

```
Demo.java:7: warning: [NamedParameters] Parameters with incorrectly labelled arguments: `// case `common`, results in a NamedParameters error.` should be `/* common= */ `
return new Foo(this) {
^
(see http://errorprone.info/bugpattern/NamedParameters)
Did you mean 'return new Foo(/* common= */this) {'?
1 warning
```

I wouldn't expect a warning about the usage of a parameter name somewhere in the body of an anonymous class. Maybe a regex should terminate on the `{` that begins the class body?

I ran into this when upgrading error-prone to 2.1.0 and running it on the Checker Framework sources.

(Also, the NamedParameters documentation is rather empty: http://errorprone.info/bugpattern/NamedParameters)