* dart2: Use the correct classname in test generation
At present test generation has a stray hard-coded reference to the pet
store Pet() class, which should reflect the actual classname under test.
* dart2: Call toDouble() in generated code for double types
At present the generated code does not correctly handle transitioning
to double when dealing with non-integer types in JSON deserialization.
This ends up with dart raising an unhandled type mismatch exception:
Unhandled exception: type 'int' is not a subtype of type 'double' where
...
Using the .toDouble() conversion when a double type is expected fixes
this up by making the typing explicit.
* dart2: Drop use of deprecated 'new' keyword in generated code
The use of the 'new' keyword in dart2 is deprecated and should be
avoided, as per the official guidance of the dart2 authors:
https://dart.dev/guides/language/effective-dart/usage#dont-use-new
* dart2: Regenerate samples for mustache template changes
