DWDS should ignore optional `offset`/`count` parameters in `getObject` request if the object has no length

[elliette]

The VM service protocol supports optional offset and count parameters in getObject.

From the VM service protocol documentation:

The offset and count parameters are used to request subranges of Instance objects with the kinds: String, List, Map, Uint8ClampedList, Uint8List, Uint16List, Uint32List, Uint64List, Int8List, Int16List, Int32List, Int64List, Flooat32List, Float64List, Inst32x3List, Float32x4List, and Float64x2List. These parameters are otherwise ignored.

However, if a client sets these optional parameters on an object that is not of one of the above listed types, DWDS throws the following exception:

getObject: NoSuchMethodError: The method '_greaterThanFromInteger' was called on null.
Receiver: null
Tried calling: _greaterThanFromInteger(0)
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:68:5)
#1      int.> (dart:core-patch/integers.dart:146:18)
#2      min (dart:math-patch/math_patch.dart:20:9)
#3      Debugger._subrange (package:dwds/src/debugging/debugger.dart:368:43)
#4      Debugger.getProperties (package:dwds/src/debugging/debugger.dart:404:25)
#5      InstanceHelper.instanceFor (package:dwds/src/debugging/instance.dart:112:47)

This is because DWDS never checks whether or not the object's length is null if count and offset are set. If the object has no length, DWDS should ignore the offset and count parameters.

Code pointers:

1. https://github.com/dart-lang/webdev/blob/master/dwds/lib/src/debugging/instance.dart#L112-L116
2. https://github.com/dart-lang/webdev/blob/master/dwds/lib/src/debugging/debugger.dart#L403-L406
3. https://github.com/dart-lang/webdev/blob/master/dwds/lib/src/debugging/debugger.dart#L368

[annagrin]

Thanks Elliott! Looks like we don't have a test for it - would you be able to add one? You can look at

webdev/dwds/test/chrome_proxy_service_test.dart

Line 398 in 29c2613

group('getObject', () {

for current examples of getObject tests.