Fix type of String.fromEnvironment() when the name is undefined.

tests/corelib/corelib.status

 # Copyright (c) 2012, 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.
 
 [ $compiler == dart2dart]
 bool_from_environment2_test: Skip
 bool_from_environment_default_value_test: Skip
 bool_from_environment_test: Skip
 int_from_environment2_test: Skip
 int_from_environment3_test: Skip
@@ skipping 128 matching lines @@
 unicode_test: Fail # inherited from VM
 
 [ $compiler == dart2dart && $minified ]
 error_stack_trace1_test: Fail # Fails in minified mode, test depends on method names.
 
 [ $compiler == dartanalyzer ]
 int_parse_radix_test: Fail, OK # Test contains errors but doesn’t mark them as expected
 list_insert_test: Fail, OK # Test contains errors but doesn’t mark them as expected
 list_removeat_test: Fail, OK # Test contains errors but doesn’t mark them as expected
 
+# Issue 16391. These tests are supposed to produce a compile time
+# error in checked mode, but they don't:
+[ $compiler == dartanalyzer && $checked ]
+from_environment_const_type_test/02: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/03: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/04: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/06: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/07: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/08: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/09: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/11: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/12: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/13: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/14: MissingCompileTimeError # Issue 16391
+from_environment_const_type_test/16: MissingCompileTimeError # Issue 16391
+from_environment_const_type_undefined_test/02: MissingCompileTimeError # Issue 16391
+from_environment_const_type_undefined_test/03: MissingCompileTimeError # Issue 16391
+from_environment_const_type_undefined_test/04: MissingCompileTimeError # Issue 16391
+from_environment_const_type_undefined_test/06: MissingCompileTimeError # Issue 16391
+from_environment_const_type_undefined_test/07: MissingCompileTimeError # Issue 16391
+from_environment_const_type_undefined_test/08: MissingCompileTimeError # Issue 16391
+
+# Analyzer's implementation of fromEnvironment assumes that undefined
+# environment variables have an unspecified value (rather than being
+# null) because it is expected that the user will supply a value when
+# the code is run.  This means that it produces slightly different
+# error messages than the VM and Dart2js.
+[ $compiler == dart2analyzer && $checked ]
+from_environment_const_type_undefined_test/09: CompileTimeError
+from_environment_const_type_undefined_test/11: CompileTimeError
+from_environment_const_type_undefined_test/12: CompileTimeError
+from_environment_const_type_undefined_test/13: CompileTimeError
+from_environment_const_type_undefined_test/14: CompileTimeError
+from_environment_const_type_undefined_test/16: CompileTimeError
+
 [ $compiler == dart2analyzer ]
 int_parse_radix_test: fail
 list_insert_test: fail
 list_removeat_test: fail
 
 [ $arch == simarm || $arch == simarm64 ]
 big_integer_parsed_*: Pass, Slow
 
 [ $arch == simmips ]
 int_parse_radix_test: Skip # Timeout
@@ skipping 14 matching lines @@
 [ $compiler == dart2js && ($runtime == safari || $runtime == safarimobilesim) ]
 list_test/01: Fail # Safari bug: Array(-2) seen as dead code.
 
 [ $runtime == ie10 ]
 num_parse_test: RuntimeError  # Issue 15316
 num_parse_test/01: RuntimeError  # Issue 15316
 
 [ $system == windows && $arch == x64 ]
 stopwatch_test: Skip  # Flaky test due to expected performance behaviour.