Implement string interpolation up to SSA.

dart/frog/leg/ssa/builder.dart

Index: dart/frog/leg/ssa/builder.dart
diff --git a/dart/frog/leg/ssa/builder.dart b/dart/frog/leg/ssa/builder.dart
index 17f103e38aa4517de513ff3b52b49a430751d93d..b02e46b440ef34800c434b94fd37abd07d7fddf7 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -927,4 +927,13 @@ class SsaBuilder implements Visitor {
     joinBlock.addPhi(phi);
     stack.add(phi);
   }
+
+  visitStringInterpolation(StringInterpolation node) {
+    compiler.unimplemented('SsaBuilder.visitStringInterpolation', node: node);
+  }
+
+  visitStringInterpolationPart(StringInterpolationPart node) {
+    compiler.unimplemented('SsaBuilder.visitStringInterpolationPart',
+                           node: node);
+  }
 }