Change code generation for dart2js to enable capturing

This CL changes the code generation for `dart2js` (and not for any other
backend) such that the variable holding the receiver of an anonymous
method invocation is declared as a normal local variable rather than
being a `Let` variable. This is needed because `dart2js` supports
capturing the former, but not the latter.

Change-Id: I054805f7006306186f2c6dfe36d7162b42293ae1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/503420
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
SLSA-Policy-Verified: SLSA Policy Verification Service <devtools-gerritcodereview-exitgate@google.com>

pkg/kernel/lib/target/targets.dart

@@ -447,6 +447,13 @@ abstract class Target {
   /// Targets can opt in to using this node for general inlining.
   bool get supportsFileUriExpression => false;
 
+  /// Whether this target supports capturing `Let` variables.
+  ///
+  /// If the target does not support capturing `Let` variables then a `Let`
+  /// expression whose variable may be captured in a nested function literal
+  /// will be lowered to a block expression with a variable declaration.
+  bool get supportsLetVariableCapture => true;
+
   /// Bit mask of [LateLowering] values for the late lowerings that should
   /// be performed by the CFE.
   ///