Fix language_2/string_interpolation_and_buffer_test.

It was badly converted to Dart 2, and was both using and testing things
that can no longer happen (like returning a non-null non-String value
from toString).

Change-Id: I085d1172f177e80c3f75af36a2028b7ea241713a
Reviewed-on: https://dart-review.googlesource.com/71144
Reviewed-by: Kevin Millikin <kmillikin@google.com>
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>

tests/language_2/language_2_dart2js.status

@@ -718,7 +718,6 @@ stacktrace_demangle_ctors_test: RuntimeError # Issue 12698
 stacktrace_rethrow_error_test/none: RuntimeError # Issue 12698
 stacktrace_rethrow_error_test/withtraceparameter: RuntimeError # Issue 12698
 stacktrace_rethrow_nonerror_test: RuntimeError # Issue 12698
-string_interpolation_and_buffer_test: RuntimeError
 string_split_test: CompileTimeError
 string_supertype_checked_test: CompileTimeError
 super_bound_closure_test/none: CompileTimeError

tests/language_2/language_2_dartdevc.status

@@ -510,7 +510,6 @@ stack_overflow_stacktrace_test: RuntimeError # Issue 29920; RangeError: Maximum
 stack_overflow_test: RuntimeError # Issue 29920; RangeError: Maximum call stack size exceeded
 stacktrace_demangle_ctors_test: RuntimeError # Issue 31089; Expect.isTrue(false) fails.
 stacktrace_test: RuntimeError # Issue 29920; Expect.isTrue(false) fails.
-string_interpolation_and_buffer_test: RuntimeError # NoSuchMethodError: method not found: '<Unexpected Null Value>'
 string_literals_test: RuntimeError # Expect.equals(expected: <\x00\x0A\x0D\x7F\xFF\u{FFFF}\u{D800}\u{DC00}\u{DBFF}\u{DFFF}>, actual: <\x00\x0A\x0D\x7F\xFF\u{FFFF}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}>) fails.
 super_test: RuntimeError # Expect.equals(expected: <0>, actual: <2>) fails.
 switch_label2_test: RuntimeError # Issue 29920; UnimplementedError: node <ShadowContinueSwitchStatement> see https://github.com/dart-lang/sdk/issues/29352 `continue #L1;

tests/language_2/string_interpolation_and_buffer_test.dart

@@ -2,77 +2,67 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// Test to ensure that StringBuffer and string interpolation behaves
-// the same and fail fast.
+// Interpolation calls `toString`.
+// The evaluation of the interpolation fails if `toString` throws or returns
+// null. In Dart 2, any method overriding `Object.toString` must return a
+// `String` or `null`. In particular, if `object.toString()` returns null, then
+// `"$object"` must not evaluate to the string `"null"`.
+//
+// The specification states that the expression of an interpolation is
+// evaluated as follows:
+//
+// 1. Evaluate $e_i$ to an object $o_i$.
+// 2. Invoke the `toString` method on *o<sub>i</sub>* with no arguments,
+//    and let *r<sub>i</sub>*$ be the returned value.
+// 3. If *r<sub>i</sub>* is not an instance of the built-in type `String`,
+//    throw an `Error`.
+//
+// (Then the resulting strings are concatenated with the literal string parts).
+//
+//
+// Adding an object to a `StringBuffer` behaves the same as evaluating
+// an expression in an interpolation. It must immediately fail if the
+// object's toString throws or returns `null`.
+//
+// This ensures that implementing interpolation via a `StringBuffer`is
+// a valid implementation choice.
 
 import "package:expect/expect.dart";
 
-class ToStringWrapper {
-  final value;
-
-  ToStringWrapper(this.value);
-
-  toString() => value;
+class ToStringString {
+  String toString() => "String";
 }
 
-wrap(value) => new ToStringWrapper(value);
-
-main() {
-  interpolate(object) {
-    var result;
-    try {
-      result = '${wrap(object)}';
-    } on ArgumentError {
-      return 'Error';
-    }
-    Expect.isTrue(result is String);
-    return 'Success';
-  }
-
-  buffer(object) {
-    var sb;
-    try {
-      sb = new StringBuffer()..write(wrap(object));
-    } on ArgumentError {
-      return 'Error';
-    }
-    if (object == null) {
-      Expect.isTrue(sb.toString() is String);
-    }
-    return 'Success';
-  }
-
-  initBuffer(object) {
-    var sb;
-    try {
-      sb = new StringBuffer(wrap(object));
-    } on ArgumentError {
-      return 'Error';
-    }
-    if (object == null) {
-      Expect.isTrue(sb.toString() is String);
-    }
-    return 'Success';
-  }
-
-  Expect.equals('Error', interpolate(null));
-  Expect.equals('Success', interpolate(""));
-  Expect.equals('Success', interpolate("string"));
-  Expect.equals('Error', interpolate([]));
-  Expect.equals('Error', interpolate([1]));
-  Expect.equals('Error', interpolate(new Object()));
-
-  Expect.equals('Error', buffer(null));
-  Expect.equals('Success', buffer(""));
-  Expect.equals('Success', buffer("string"));
-  Expect.equals('Error', buffer([]));
-  Expect.equals('Error', buffer([1]));
-  Expect.equals('Error', buffer(new Object()));
-
-  Expect.equals('Error', initBuffer(null));
-  Expect.equals('Success', initBuffer(""));
-  Expect.equals('Success', initBuffer("string"));
-  Expect.equals('Error', initBuffer([]));
-  Expect.equals('Error', initBuffer([1]));
-  Expect.equals('Error', initBuffer(new Object()));
+class ToStringNull {
+  String toString() => null;
+}
+
+class ToStringThrows {
+  String toString() => throw "Throw";
+}
+
+void main() {
+  var s = ToStringString();
+  var n = ToStringNull();
+  var t = ToStringThrows();
+
+  Expect.equals("$s$s", "StringString");
+  // Throws immediately when evaluating the first interpolated expression.
+  Expect.throws<String>(() => "$t${throw "unreachable"}", (e) => e == "Throw");
+  Expect.throws<Error>(() => "$n${throw "unreachable"}");
+
+  // Throws immediately when adding object that doesn't return a String.
+  Expect.equals(
+      (StringBuffer()..write(s)..write(s)).toString(), "StringString");
+  Expect.throws<String>(
+      () => StringBuffer()..write(t)..write(throw "unreachable"),
+      (e) => e == "Throw");
+  Expect.throws<Error>(
+      () => StringBuffer()..write(n)..write(throw "unreachable"));
+
+  // Same behavior for constructor argument as if adding it to buffer later.
+  Expect.equals((StringBuffer(s)..write(s)).toString(), "StringString");
+  Expect.throws<String>(
+      () => StringBuffer(t)..write(throw "unreachable"), (e) => e == "Throw");
+  Expect.throws<Error>(() => StringBuffer(n)..write(throw "unreachable"));
 }