[ddc] Fix non-null assertion crash

`VariableDeclaration.name` could be null when the variable
was synthetically introduced by the compiler. In this case the
expression compiler can ignore the variable because it's value
doesn't reflect the value of a variable in the original dart
source code.

Add a regression test for the expression compiler that
ensures code that introduces a synthetic variable will not
cause a crash when you stop at a breakpoint.

Fixes: dart-lang/sdk#49373
Change-Id: Ifc5f72ab4e5733ead17a20889c222bcc46921f6b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/250343
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Anna Gringauze <annagrin@google.com>

Author: nshahan

pkg/dev_compiler/lib/src/kernel/expression_compiler.dart

@@ -163,13 +163,17 @@ class DartScopeBuilder extends Visitor<void> with VisitorVoidMixin {
 
   @override
   void visitVariableDeclaration(VariableDeclaration decl) {
+    var name = decl.name;
     // Collect locals and formals appearing before current breakpoint.
     // Note that we include variables with no offset because the offset
     // is not set in many cases in generated code, so omitting them would
     // make expression evaluation fail in too many cases.
     // Issue: https://github.com/dart-lang/sdk/issues/43966
-    if (decl.fileOffset < 0 || decl.fileOffset < _offset) {
-      _definitions[decl.name!] = decl.type;
+    //
+    // A null name signals that the variable was synthetically introduced by the
+    // compiler so they are skipped.
+    if ((decl.fileOffset < 0 || decl.fileOffset < _offset) && name != null) {
+      _definitions[name] = decl.type;
     }
     super.visitVariableDeclaration(decl);
   }

pkg/dev_compiler/test/expression_compiler/expression_compiler_e2e_shared.dart

@@ -356,6 +356,35 @@ void runNullSafeSharedTests(SetupCompilerOptions setup, TestDriver driver) {
           expectedResult: 'true');
     });
   });
+
+  group('Synthetic variables', () {
+    var source = r'''
+      dynamic couldReturnNull() => null;
+
+      main() {
+        var i = couldReturnNull() ?? 10;
+        // Breakpoint: bp
+        print(i);
+      }
+        ''';
+
+    setUpAll(() async {
+      await driver.initSource(setup, source);
+    });
+
+    tearDownAll(() async {
+      await driver.cleanupTest();
+    });
+
+    test('do not cause a crash the expression compiler', () async {
+      // The null aware code in the test source causes the compiler to introduce
+      // a let statement that includes a synthetic variable declaration.
+      // That variable has no name and was causing a crash in the expression
+      // compiler https://github.com/dart-lang/sdk/issues/49373.
+      await driver.check(
+          breakpointId: 'bp', expression: 'true', expectedResult: 'true');
+    });
+  });
 }
 
 /// Shared tests that are valid in legacy (before 2.12) and are agnostic to