[dart2wasm] Emit unreachable as default case when switch is exhaustive

The front-end variable initialization analysis considers a variable
definitely initialized if it's initialized in all `case` branches of an
"explicitly exhaustive" `switch` statement, i.e. a `switch` statement switching on an enum value where the `case` branches cover all variants
of the enum.

In order for the emitted Wasm code to validate according to the
flow-sensitive non-nullable locals validation scheme being discussed in
https://github.com/WebAssembly/function-references/issues/44 this
definite initialization must be reflected in the static control flow of
the Wasm code. Thus, such a switch statement must not have static
control flow exiting the switch statement through the (dynamically
unreachable) default case.

Change-Id: I53628d2c6e9690214ffb4c60498c4f2b895f33e7
Cq-Include-Trybots: luci.dart.try:dart2wasm-linux-x64-d8-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/245985
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Aske Simon Christensen <askesc@google.com>

pkg/dart2wasm/lib/code_generator.dart

@@ -990,9 +990,15 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
         }
       }
     }
-    w.Label defaultLabel =
-        defaultCase != null ? switchLabels[defaultCase]! : doneLabel;
-    b.br(defaultLabel);
+
+    // No explicit cases matched
+    if (node.isExplicitlyExhaustive) {
+      b.unreachable();
+    } else {
+      w.Label defaultLabel =
+          defaultCase != null ? switchLabels[defaultCase]! : doneLabel;
+      b.br(defaultLabel);
+    }
 
     // Emit case bodies
     for (SwitchCase c in node.cases) {