Since MethodCall equality checks are limited to test scenarios, this
patch replaces them with an equivalent test matcher. At present
MethodCalls are always used in scenarios where indentity-based
equality/hashing is appropriate.

This change avoids an assertion failure when MethodCall args are
Iterable (possible since args are of type dyanmic), and hashValue() from
dart:ui asserts that its input is not an Iterable.

The alternative of implementing support for deep equality in dart:ui was
rejected on the basis that if we're to encourage performant code,
expensive checks should be obviously-expensive to the author.