[CFE, Patterns] "Null check operator used on a null value" at InferenceVisitorImpl.inferMapEntry

[alexmarkov]

CFE crashes on the following tests:

co19/LanguageFeatures/Patterns/scope_A02_t04
co19/LanguageFeatures/Patterns/scope_A02_t09

Example log:

Crash when compiling file:///b/s/w/ir/tests/co19/src/LanguageFeatures/Patterns/scope_A02_t04.dart at character offset 1202:
Null check operator used on a null value

#0      InferenceVisitorImpl.inferMapEntry (package:front_end/src/fasta/type_inference/inference_visitor.dart:4232:54)
#1      InferenceVisitorImpl.visitMapLiteral (package:front_end/src/fasta/type_inference/inference_visitor.dart:4423:31)
#2      MapLiteral.accept1 (package:kernel/ast.dart:8822:9)
#3      InferenceVisitorImpl._inferExpression (package:front_end/src/fasta/type_inference/inference_visitor.dart:264:27)
#4      InferenceVisitorImpl.inferExpression (package:front_end/src/fasta/type_inference/inference_visitor.dart:299:40)
#5      InferenceVisitorImpl.visitVariableDeclaration (package:front_end/src/fasta/type_inference/inference_visitor.dart:8702:11)
#6      VariableDeclaration.accept (package:kernel/ast.dart:11342:43)
#7      InferenceVisitorImpl.inferStatement (package:front_end/src/fasta/type_inference/inference_visitor.dart:242:26)
#8      InferenceVisitorImpl._visitStatements (package:front_end/src/fasta/type_inference/inference_visitor.dart:852:50)
#9      InferenceVisitorImpl.visitBlock (package:front_end/src/fasta/type_inference/inference_visitor.dart:873:31)
#10     Block.accept (package:kernel/ast.dart:9755:43)
#11     InferenceVisitorImpl.inferStatement (package:front_end/src/fasta/type_inference/inference_visitor.dart:242:26)
#12     TypeInferrerImpl.inferFunctionBody (package:front_end/src/fasta/type_inference/type_inferrer.dart:196:17)
#13     BodyBuilder.finishFunction (package:front_end/src/fasta/kernel/body_builder.dart:1344:43)
#14     DietListener.buildFunctionBody (package:front_end/src/fasta/source/diet_listener.dart:1123:19)
#15     DietListener.endTopLevelMethod (package:front_end/src/fasta/source/diet_listener.dart:376:5)
#16     Parser.parseTopLevelMethod (package:_fe_analyzer_shared/src/parser/parser_impl.dart:3684:14)
#17     Parser.parseTopLevelMemberImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:3426:14)
#18     Parser.parseTopLevelDeclarationImpl (package:_fe_analyzer_shared/src/parser/parser_impl.dart:608:14)
#19     Parser.parseUnit (package:_fe_analyzer_shared/src/parser/parser_impl.dart:405:15)
#20     SourceLoader.buildBody (package:front_end/src/fasta/source/source_loader.dart:1267:12)
<asynchronous suspension>
#21     SourceLoader.buildBodies (package:front_end/src/fasta/source/source_loader.dart:676:7)
<asynchronous suspension>
#22     KernelTarget.buildComponent.<anonymous closure> (package:front_end/src/fasta/kernel/kernel_target.dart:611:7)
<asynchronous suspension>
#23     withCrashReporting (package:front_end/src/fasta/crash.dart:136:12)
<asynchronous suspension>
#24     _buildInternal (package:front_end/src/kernel_generator_impl.dart:206:19)
<asynchronous suspension>
#25     withCrashReporting (package:front_end/src/fasta/crash.dart:136:12)
<asynchronous suspension>
#26     generateKernel.<anonymous closure> (package:front_end/src/kernel_generator_impl.dart:48:12)
<asynchronous suspension>
#27     generateKernel (package:front_end/src/kernel_generator_impl.dart:47:10)
<asynchronous suspension>
#28     kernelForModule (package:front_end/src/api_prototype/kernel_generator.dart:100:11)
<asynchronous suspension>
#29     SingleShotCompilerWrapper.compileInternal (file:///b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:407:11)
<asynchronous suspension>
#30     Compiler.compile.<anonymous closure> (file:///b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:215:45)
<asynchronous suspension>
#31     _processLoadRequest (file:///b/s/w/ir/cache/builder/sdk/pkg/vm/bin/kernel_service.dart:889:37)
<asynchronous suspension>

--- Re-run this test:
python3 tools/test.py -n dartk-strong-linux-release-x64 co19/LanguageFeatures/Patterns/scope_A02_t04

@johnniwinther @chloestefantsova