feat(material/theming): Add APIs to check what information theme has

Author: mmalerba

src/material/_index.scss

@@ -146,4 +146,4 @@ list-density, list-base;
 // New theming APIs, currently in development:
 @forward './core/theming/inspection' as private-* show private-get-theme-version,
   private-get-theme-type, private-get-theme-color, private-get-theme-typography,
-  private-get-theme-density;
+  private-get-theme-density, private-theme-has;

src/material/core/theming/_inspection.scss

@@ -53,6 +53,9 @@ $_typography-properties: (font, font-family, line-height, font-size, letter-spac
     // TODO(mmalerba): implement for old style theme objects.
     @error #{'get-theme-type does not support legacy theme objects.'};
   }
+  @if not theme-has($theme, color) {
+    @error 'Color information is not available on this theme.';
+  }
   @return map.get($theme, $_internals, theme-type) or light;
 }
 
@@ -86,6 +89,9 @@ $_typography-properties: (font, font-family, line-height, font-size, letter-spac
     // TODO(mmalerba): implement for old style theme objects.
     @error #{'get-theme-color does not support legacy theme objects.'};
   }
+  @if not theme-has($theme, color) {
+    @error 'Color information is not available on this theme.';
+  }
   $color-roles: map.get($theme, $_internals, color-tokens, (mdc, theme));
   $result: map.get($color-roles, $color-role-name);
   @if not $result {
@@ -105,6 +111,9 @@ $_typography-properties: (font, font-family, line-height, font-size, letter-spac
     // TODO(mmalerba): implement for old style theme objects.
     @error #{'get-theme-color does not support legacy theme objects.'};
   }
+  @if not theme-has($theme, color) {
+    @error 'Color information is not available on this theme.';
+  }
   $palettes: map.get($theme, $_internals, palettes);
   $palette: map.get($palettes, $palette-name);
   @if not $palette {
@@ -131,6 +140,9 @@ $_typography-properties: (font, font-family, line-height, font-size, letter-spac
     // TODO(mmalerba): implement for old style theme objects.
     @error #{'get-theme-typography does not support legacy theme objects.'};
   }
+  @if not theme-has($theme, typography) {
+    @error 'Typography information is not available on this theme.';
+  }
   @if not list.index($_m3-typescales, $typescale) {
     @error #{'Valid typescales are: #{$_m3-typescales}. Got:'} $typescale;
   }
@@ -158,9 +170,39 @@ $_typography-properties: (font, font-family, line-height, font-size, letter-spac
     // TODO(mmalerba): implement for old style theme objects.
     @error #{'get-theme-density does not support legacy theme objects.'};
   }
+  @if not theme-has($theme, density) {
+    @error 'Density information is not available on this theme.';
+  }
   @return map.get($theme, $_internals, density-scale);
 }
 
+/// Checks whether the theme has information about given theming system.
+/// @param {Map} $theme The theme
+/// @param {String} $system The system to check
+/// @param {Boolean} Whether the theme has information about the system.
+@function theme-has($theme, $system) {
+  $err: _validate-theme-object($theme);
+  @if $err {
+    // TODO(mmalerba): implement for old style theme objects.
+    @error #{'get-theme-density does not support legacy theme objects.'};
+  }
+  @if $system == base {
+    @return map.get($theme, $_internals, base-tokens) != null;
+  }
+  @if $system == color {
+    @return map.get($theme, $_internals, color-tokens) != null and
+        map.get($theme, $_internals, theme-type) != null and
+        map.get($theme, $_internals, palettes) != null;
+  }
+  @if $system == typography {
+    @return map.get($theme, $_internals, typography-tokens) != null;
+  }
+  @if $system == density {
+    @return map.get($theme, $_internals, density-scale) != null
+  }
+  @error 'Valid systems are: base, color, typography, density. Got:' $system;
+}
+
 /// Gets the set of tokens from the given theme, limited to those affected by the requested theming
 /// systems.
 /// @param {Map} $theme The theme to get tokens from.