Make clear that const constructors don't always create constant objec…

…ts (#1119)

examples/misc/test/language_tour/classes_test.dart

@@ -168,4 +168,15 @@ void main() {
 
     expect(pointAndLine1 == pointAndLine2, isTrue);
   });
+
+  test('nonconst_const_constructor', () {
+    // #docregion nonconst-const-constructor
+    var a = const ImmutablePoint(1, 1); // Creates a constant
+    var b = ImmutablePoint(1, 1); // Does NOT create a constant
+
+    assert(!identical(a, b)); // NOT the same instance!
+    // #enddocregion nonconst-const-constructor
+
+    expect(a == b, isFalse);
+  });
 }

src/_guides/language/language-tour.md

@@ -2308,12 +2308,12 @@ var p2 = new Point.fromJson({'x': 1, 'y': 2});
 {% endprettify %}
 
 <aside class="alert alert-info" markdown="1">
-**Version note:** The `new` keyword became optional in Dart 2.
+  **Version note:** The `new` keyword became optional in Dart 2.
 </aside>
 
-Some classes provide constant constructors. To create a compile-time
-constant using a constant constructor, put the `const` keyword
-before the constructor name:
+Some classes provide [constant constructors](#constant-constructors).
+To create a compile-time constant using a constant constructor,
+put the `const` keyword before the constructor name:
 
 <?code-excerpt "misc/test/language_tour/classes_test.dart (const)"?>
 {% prettify dart %}
@@ -2354,9 +2354,21 @@ const pointAndLine = {
 };
 {% endprettify %}
 
+If a constant constructor is outside of a constant context
+and is invoked without `const`,
+it creates a **non-constant object**:
+
+<?code-excerpt "misc/test/language_tour/classes_test.dart (nonconst-const-constructor)"?>
+{% prettify dart %}
+var a = const ImmutablePoint(1, 1); // Creates a constant
+var b = ImmutablePoint(1, 1); // Does NOT create a constant
+
+assert(!identical(a, b)); // NOT the same instance!
+{% endprettify %}
+
 <aside class="alert alert-info" markdown="1">
-**Version note:** The `const` keyword became optional
-within a constant context in Dart 2.
+  **Version note:** The `const` keyword became optional
+  within a constant context in Dart 2.
 </aside>
 
 
@@ -2694,6 +2706,11 @@ class ImmutablePoint {
 }
 {% endprettify %}
 
+Constant constructors don't always create constants.
+For details, see the section on
+[using constructors](#using-constructors).
+
+
 #### Factory constructors
 
 Use the `factory` keyword when implementing a constructor that doesn’t