[CFE] dart2js/mixin_super/main fuzzing failure

[jensjoha]

Running these two:

out/ReleaseX64/dart --enable-asserts pkg/front_end/test/fasta/strong_suite.dart -DsemiFuzz=true -- strong/dart2js/mixin_super/...
out/ReleaseX64/dart --enable-asserts pkg/front_end/test/fasta/weak_suite.dart -DsemiFuzz=true -- weak/dart2js/mixin_super/...

gives e.g.:

Compiled and got 2 libs
 -> invalidating [...]/pkg/front_end/testcases/dart2js/mixin_super/main.dart
 -> and got 2 libs
Sets {_Class4b&Class4a&Mixin2.field1, _Class4b&Class4a&Mixin2.field2, _Class4b&Class4a&Mixin2.field2, _Class4b&Class4a&Mixin2.property2, _Class4b&Class4a&Mixin2.property2, _Class4b&Class4a&Mixin2.method2, _Class4b&Class4a&Mixin2.method1, _Class4b&Class4a&Mixin2.property1, _Class4b&Class4a&Mixin2.property1, _Class4b&Class4a&Mixin2.field1} and {_Class4b&Class4a&Mixin2.property2, _Class4b&Class4a&Mixin2.property2, _Class4b&Class4a&Mixin2.field1, _Class4b&Class4a&Mixin2.field2, _Class4b&Class4a&Mixin2.field2, _Class4b&Class4a&Mixin2._#Mixin2#field1#AI, _Class4b&Class4a&Mixin2._#Mixin2#field1#AI, _Class4b&Class4a&Mixin2._#Mixin2#field2#AI, _Class4b&Class4a&Mixin2._#Mixin2#field2#AI, _Class4b&Class4a&Mixin2.method2, _Class4b&Class4a&Mixin2.method1, _Class4b&Class4a&Mixin2.property1, _Class4b&Class4a&Mixin2.property1, _Class4b&Class4a&Mixin2.field1} are not equivalent
.root
 Library(library [...]/pkg/front_end/testcases/dart2js/mixin_super/main.dart).classes[12]
  Class(_Class4b&Class4a&Mixin2).procedures.length
Sets {_Class5b&Class5a&Mixin2.field1, _Class5b&Class5a&Mixin2.field2, _Class5b&Class5a&Mixin2.field2, _Class5b&Class5a&Mixin2.property2, _Class5b&Class5a&Mixin2.property2, _Class5b&Class5a&Mixin2.method2, _Class5b&Class5a&Mixin2.method1, _Class5b&Class5a&Mixin2.property1, _Class5b&Class5a&Mixin2.property1, _Class5b&Class5a&Mixin2.field1} and {_Class5b&Class5a&Mixin2.property2, _Class5b&Class5a&Mixin2.property2, _Class5b&Class5a&Mixin2.field1, _Class5b&Class5a&Mixin2.field2, _Class5b&Class5a&Mixin2.field2, _Class5b&Class5a&Mixin2._#Mixin2#field1#AI, _Class5b&Class5a&Mixin2._#Mixin2#field1#AI, _Class5b&Class5a&Mixin2._#Mixin2#field2#AI, _Class5b&Class5a&Mixin2._#Mixin2#field2#AI, _Class5b&Class5a&Mixin2.method2, _Class5b&Class5a&Mixin2.method1, _Class5b&Class5a&Mixin2.property1, _Class5b&Class5a&Mixin2.property1, _Class5b&Class5a&Mixin2.field1} are not equivalent
.root
 Library(library [...]/pkg/front_end/testcases/dart2js/mixin_super/main.dart).classes[15]
  Class(_Class5b&Class5a&Mixin2).procedures.length
Sets {_Class6b&Class6a&Mixin2.field1, _Class6b&Class6a&Mixin2.field2, _Class6b&Class6a&Mixin2.field2, _Class6b&Class6a&Mixin2.property2, _Class6b&Class6a&Mixin2.property2, _Class6b&Class6a&Mixin2.method2, _Class6b&Class6a&Mixin2.method1, _Class6b&Class6a&Mixin2.property1, _Class6b&Class6a&Mixin2.property1, _Class6b&Class6a&Mixin2.field1} and {_Class6b&Class6a&Mixin2.property2, _Class6b&Class6a&Mixin2.property2, _Class6b&Class6a&Mixin2.field1, _Class6b&Class6a&Mixin2.field2, _Class6b&Class6a&Mixin2.field2, _Class6b&Class6a&Mixin2._#Mixin2#field1#AI, _Class6b&Class6a&Mixin2._#Mixin2#field1#AI, _Class6b&Class6a&Mixin2._#Mixin2#field2#AI, _Class6b&Class6a&Mixin2._#Mixin2#field2#AI, _Class6b&Class6a&Mixin2.method2, _Class6b&Class6a&Mixin2.method1, _Class6b&Class6a&Mixin2.property1, _Class6b&Class6a&Mixin2.property1, _Class6b&Class6a&Mixin2.field1} are not equivalent
.root
 Library(library [...]/pkg/front_end/testcases/dart2js/mixin_super/main.dart).classes[18]
  Class(_Class6b&Class6a&Mixin2).procedures.length


Fuzzed component changed in an unexpected way.
1 failed:
strong/dart2js/mixin_super/main: SemiFuzzFailure

Where on the recompile seemingly there's suddenly these extra entries in a class (etc):

_Class4b&Class4a&Mixin2._#Mixin2#field1#AI
_Class4b&Class4a&Mixin2._#Mixin2#field1#AI
_Class4b&Class4a&Mixin2._#Mixin2#field2#AI
_Class4b&Class4a&Mixin2._#Mixin2#field2#AI

/cc @johnniwinther