Allow implicit dynamic to be inferred from explicit dynamic

[nex3]

Consider the following code:

class Foo {
  List<dynamic> values;

  Foo(Iterable<dynamic> values)
      : values = new List.unmodifiable(values);
}

When I run dartanalyzer --strong --no-implicit-dynamic on this, I get:

[error] Missing type arguments for generic type 'List<dynamic>'. Either add an explicit type like 'dynamic', or enable implicit-dynamic in your Analyzer options.

This seems counterproductive. I'm explicitly declaring types at my API boundaries, and I expect them to be inferred elsewhere, just like they would be if I'd written <String> instead of <dynamic>.

[jmesserly]

merging into #26781