Add validation for Flutter constraints.

lib/src/validator/sdk_constraint.dart

Index: lib/src/validator/sdk_constraint.dart
diff --git a/lib/src/validator/sdk_constraint.dart b/lib/src/validator/sdk_constraint.dart
index bdf7e7be8bf98309738a9e77e1561546cde6c208..aa57baaa31f89e54aaf2751a61523e4e9485bd10 100644
--- a/lib/src/validator/sdk_constraint.dart
+++ b/lib/src/validator/sdk_constraint.dart
@@ -4,9 +4,15 @@
 
 import 'dart:async';
 
+import 'package:pub_semver/pub_semver.dart';
+
 import '../entrypoint.dart';
 import '../validator.dart';
 
+/// The range of all Dart SDK versions that don't support Flutter SDK
+/// constraints.
+final _preFlutterSupport = new VersionConstraint.parse("<1.19.0");
+
 /// A validator that validates that a package's SDK constraint doesn't use the
 /// "^" syntax.
 class SdkConstraintValidator extends Validator {
@@ -14,15 +20,33 @@ class SdkConstraintValidator extends Validator {
     : super(entrypoint);
 
   Future validate() async {
-    var constraint = entrypoint.root.pubspec.dartSdkConstraint;
-    if (!constraint.toString().startsWith("^")) return;
-
-    errors.add(
-        "^ version constraints aren't allowed for SDK constraints since "
-          "older versions of pub don't support them.\n"
-        "Expand it manually instead:\n"
-        "\n"
-        "environment:\n"
-        "  sdk: \">=${constraint.min} <${constraint.max}\"");
+    var dartConstraint = entrypoint.root.pubspec.dartSdkConstraint;
+    if (dartConstraint.toString().startsWith("^")) {
+      errors.add(
+          "^ version constraints aren't allowed for SDK constraints since "
+            "older versions of pub don't support them.\n"
+          "Expand it manually instead:\n"
+          "\n"
+          "environment:\n"
+          "  sdk: \">=${dartConstraint.min} <${dartConstraint.max}\"");
+    }
+
+    if (entrypoint.root.pubspec.flutterSdkConstraint != null &&
+        dartConstraint.allowsAny(_preFlutterSupport)) {
+      var newDartConstraint = dartConstraint.difference(_preFlutterSupport);
+      if (newDartConstraint.isEmpty ||
+          newDartConstraint ==
+              VersionConstraint.any.difference(_preFlutterSupport)) {
+        newDartConstraint = new VersionConstraint.parse("<2.0.0")
+            .difference(_preFlutterSupport);
+      }
+
+      errors.add(
+          "Older versions of pub don't support Flutter SDK constraints.\n"
+          "Make sure your SDK constraint excludes those old versions:\n"
+          "\n"
+          "environment:\n"
+          "  sdk: \"$newDartConstraint\"");
+    }
   }
 }