Fix analyzer's handling of import prefixes not followed by '.'.

tests/language/if_null_assignment_behavior_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 // Verify semantics of the ??= operator, including order of operations, by
 // keeping track of the operations performed.
 
 // SharedOptions=--enable-null-aware-operators
 
 import "package:expect/expect.dart";
@@ skipping 152 matching lines @@
   // v ??= e is equivalent to ((x) => x == null ? v = e : x)(v)
   xGetValue = 1; check(1, () => x ??= bad(), ['x']); /// 07: ok
   yGetValue = 1; check(1, () => x ??= y, ['x', 'y', 'x=1']); /// 08: ok
   h.xGetValue = 1; check(1, () => h.x ??= bad(), ['h.x']); /// 09: ok
   yGetValue = 1; check(1, () => h.x ??= y, ['h.x', 'y', 'h.x=1']); /// 10: ok
   { var l = 1; check(1, () => l ??= bad(), []); } /// 11: ok
   { var l; yGetValue = 1; check(1, () => l ??= y, ['y']); Expect.equals(1, l); } /// 12: ok
   { final l = 1; check(1, () => l ??= bad(), []); } /// 13: static type warning
   { final l = null; yGetValue = 1; checkThrows(noMethod, () => l ??= y, ['y']); } /// 14: static type warning
   check(C, () => C ??= bad(), []); /// 15: static type warning
+  h ??= null; /// 29: compile-time error
+  h[0] ??= null; /// 30: compile-time error
 
   // C.v ??= e is equivalent to ((x) => x == null ? C.v = e : x)(C.v)
   C.xGetValue = 1; check(1, () => C.x ??= bad(), ['C.x']); /// 16: ok
   yGetValue = 1; check(1, () => C.x ??= y, ['C.x', 'y', 'C.x=1']); /// 17: ok
   h.C.xGetValue = 1; check(1, () => h.C.x ??= bad(), ['h.C.x']); /// 18: ok
   yGetValue = 1; check(1, () => h.C.x ??= y, ['h.C.x', 'y', 'h.C.x=1']); /// 19: ok
 
   // e1.v ??= e2 is equivalent to
   // ((x) => ((y) => y == null ? x.v = e2 : y)(x.v))(e1)
   xGetValue = new C('x'); xGetValue.vGetValue = 1; /// 20: ok
@@ skipping 12 matching lines @@
   xGetValue = new C('x'); yGetValue = 1; zGetValue = 2;          /// 25: ok
   check(2, () => x[y] ??= z, ['x', 'y', 'x[1]', 'z', 'x[1]=2']); /// 25: continued
 
   // e1?.v ??= e2 is equivalent to ((x) => x == null ? null : x.v ??= e2)(e1).
   check(null, () => x?.v ??= bad(), ['x']); /// 26: ok
   xGetValue = new C('x'); xGetValue.vGetValue = 1; /// 27: ok
   check(1, () => x?.v ??= bad(), ['x', 'x.v']);     /// 27: continued
   xGetValue = new C('x'); yGetValue = 1;                 /// 28: ok
   check(1, () => x?.v ??= y, ['x', 'x.v', 'y', 'x.v=1']); /// 28: continued
 }