Implement dput checks robustly

[vsmenon]

From @vsmenon on May 8, 2015 16:58

dput needs to check:

• NSM for non-existent fields (for JS types too?)
• Writability (final fields, methods, no setter)
• Type check - value passed in must be a subtype of field / setter type

dload needs to check:

• NSM for non-existent fields (for JS types too?)

Copied from original issue: dart-archive/dev_compiler#170

[jacob314]

Updated checkboxes